*converter: make --plugin-dir accept the actual root plugin directory.

For some reason, it was adding also the magnum / magnum-d directory,
which isn't really useful, especially in cases where the directory is
not at all or it's desired to pick a debug plugin from a release
executable and vice versa.

Also the distancefieldconverter was still attempting to join with an
absolute path -- somehow 7fb63a9434 missed
this one.

I really need to write regression tests for all this, sigh.

src/Magnum/SceneTools/sceneconverter.cpp

@@ -337,12 +337,12 @@ is specified as well, the IDs reference attributes of the first mesh.)")
     /* Importer manager */
     PluginManager::Manager<Trade::AbstractImporter> importerManager{
         args.value("plugin-dir").empty() ? Containers::String{} :
-        Utility::Path::join(args.value("plugin-dir"), Trade::AbstractImporter::pluginSearchPaths().back())};
+        Utility::Path::join(args.value("plugin-dir"), Utility::Path::split(Trade::AbstractImporter::pluginSearchPaths().back()).second())};
 
     /* Scene converter manager */
     PluginManager::Manager<Trade::AbstractSceneConverter> converterManager{
         args.value("plugin-dir").empty() ? Containers::String{} :
-        Utility::Path::join(args.value("plugin-dir"), Trade::AbstractSceneConverter::pluginSearchPaths().back())};
+        Utility::Path::join(args.value("plugin-dir"), Utility::Path::split(Trade::AbstractSceneConverter::pluginSearchPaths().back()).second())};
 
     Containers::Pointer<Trade::AbstractImporter> importer = importerManager.loadAndInstantiate(args.value("importer"));
     if(!importer) {

src/Magnum/ShaderTools/shaderconverter.cpp

@@ -359,7 +359,7 @@ see documentation of a particular converter for more information.)")
     /* Set up a converter manager */
     PluginManager::Manager<ShaderTools::AbstractConverter> converterManager{
         args.value("plugin-dir").empty() ? Containers::String{} :
-        Utility::Path::join(args.value("plugin-dir"), ShaderTools::AbstractConverter::pluginSearchPaths().back())};
+        Utility::Path::join(args.value("plugin-dir"), Utility::Path::split(ShaderTools::AbstractConverter::pluginSearchPaths().back()).second())};
 
     /* Data passed from one converter to another in case there's more than one */
     Containers::Array<char> data;

src/Magnum/Text/fontconverter.cpp

@@ -23,6 +23,7 @@
     DEALINGS IN THE SOFTWARE.
 */
 
+#include <Corrade/Containers/Pair.h>
 #include <Corrade/PluginManager/Manager.h>
 #include <Corrade/Utility/Arguments.h>
 #include <Corrade/Utility/DebugStl.h>
@@ -158,12 +159,12 @@ int FontConverter::exec() {
     /* Font converter dependencies */
     PluginManager::Manager<Trade::AbstractImageConverter> imageConverterManager{
         args.value("plugin-dir").empty() ? Containers::String{} :
-        Utility::Path::join(args.value("plugin-dir"), Trade::AbstractImageConverter::pluginSearchPaths().back())};
+        Utility::Path::join(args.value("plugin-dir"), Utility::Path::split(Trade::AbstractImageConverter::pluginSearchPaths().back()).second())};
 
     /* Load font */
     PluginManager::Manager<Text::AbstractFont> fontManager{
         args.value("plugin-dir").empty() ? Containers::String{} :
-        Utility::Path::join(args.value("plugin-dir"), Text::AbstractFont::pluginSearchPaths().back())};
+        Utility::Path::join(args.value("plugin-dir"), Utility::Path::split(Text::AbstractFont::pluginSearchPaths().back()).second())};
     Containers::Pointer<Text::AbstractFont> font = fontManager.loadAndInstantiate(args.value("font"));
     if(!font) return 1;
 
@@ -171,7 +172,7 @@ int FontConverter::exec() {
        (MagnumFontConverter needs TgaImageConverter, for example) */
     PluginManager::Manager<Text::AbstractFontConverter> converterManager{
         args.value("plugin-dir").empty() ? Containers::String{} :
-        Utility::Path::join(args.value("plugin-dir"), Text::AbstractFontConverter::pluginSearchPaths().back())};
+        Utility::Path::join(args.value("plugin-dir"), Utility::Path::split(Text::AbstractFontConverter::pluginSearchPaths().back()).second())};
     converterManager.registerExternalManager(imageConverterManager);
 
     /* Load font converter */

src/Magnum/TextureTools/distancefieldconverter.cpp

@@ -24,6 +24,7 @@
 */
 
 #include <Corrade/Containers/Optional.h>
+#include <Corrade/Containers/Pair.h>
 #include <Corrade/Utility/Arguments.h>
 #include <Corrade/Utility/DebugStl.h> /** @todo remove once Arguments is std::string-free */
 #include <Corrade/Utility/Path.h>
@@ -156,14 +157,14 @@ int DistanceFieldConverter::exec() {
     /* Load importer plugin */
     PluginManager::Manager<Trade::AbstractImporter> importerManager{
         args.value("plugin-dir").empty() ? Containers::String{} :
-        Utility::Path::join(args.value("plugin-dir"), Trade::AbstractImporter::pluginSearchPaths()[0])};
+        Utility::Path::join(args.value("plugin-dir"), Utility::Path::split(Trade::AbstractImporter::pluginSearchPaths().back()).second())};
     Containers::Pointer<Trade::AbstractImporter> importer = importerManager.loadAndInstantiate(args.value("importer"));
     if(!importer) return 1;
 
     /* Load converter plugin */
     PluginManager::Manager<Trade::AbstractImageConverter> converterManager{
         args.value("plugin-dir").empty() ? Containers::String{} :
-        Utility::Path::join(args.value("plugin-dir"), Trade::AbstractImageConverter::pluginSearchPaths()[0])};
+        Utility::Path::join(args.value("plugin-dir"), Utility::Path::split(Trade::AbstractImageConverter::pluginSearchPaths().back()).second())};
     Containers::Pointer<Trade::AbstractImageConverter> converter = converterManager.loadAndInstantiate(args.value("converter"));
     if(!converter) return 2;
 

src/Magnum/Trade/imageconverter.cpp

@@ -25,6 +25,7 @@
 
 #include <Corrade/Containers/Optional.h>
 #include <Corrade/Containers/GrowableArray.h>
+#include <Corrade/Containers/Pair.h>
 #include <Corrade/Containers/StaticArray.h>
 #include <Corrade/Containers/StridedArrayView.h>
 #include <Corrade/PluginManager/Manager.h>
@@ -447,7 +448,7 @@ no -C / --converter is specified, AnyImageConverter is used.)")
 
     PluginManager::Manager<Trade::AbstractImporter> importerManager{
         args.value("plugin-dir").empty() ? Containers::String{} :
-        Utility::Path::join(args.value("plugin-dir"), Trade::AbstractImporter::pluginSearchPaths().back())};
+        Utility::Path::join(args.value("plugin-dir"), Utility::Path::split(Trade::AbstractImporter::pluginSearchPaths().back()).second())};
 
     const Int dimensions = args.value<Int>("dimensions");
     /** @todo make them array options as well? */
@@ -921,7 +922,7 @@ no -C / --converter is specified, AnyImageConverter is used.)")
 
     PluginManager::Manager<Trade::AbstractImageConverter> converterManager{
         args.value("plugin-dir").empty() ? Containers::String{} :
-        Utility::Path::join(args.value("plugin-dir"), Trade::AbstractImageConverter::pluginSearchPaths().back())};
+        Utility::Path::join(args.value("plugin-dir"), Utility::Path::split(Trade::AbstractImageConverter::pluginSearchPaths().back()).second())};
 
     /* Assume there's always one passed --converter option less, and the last
        is implicitly AnyImageConverter. All converters except the last one are