Platform: make it possible to override DPI scaling from the app.

This was sadly broken since probably ever. Since it was hard to wrap my
head around this, added also some code to test this.

doc/changelog.dox

@@ -268,6 +268,9 @@ See also:
 -   Calling @ref Platform::Sdl2Application::setSwapInterval() with @cpp 0 @ce
     caused @ref Platform::Sdl2Application::setMinimalLoopPeriod() "setMinimalLoopPeriod()"
     to be ignored even though Vsync was in fact not enabled.
+-   It was not possible to override DPI scaling using
+    @ref Platform::Sdl2Application::Configuration as command-line arguments
+    always got a priority (see [mosra/magnum#416](https://github.com/mosra/magnum/issues/416))
 -   Fixed an otherwise harmless OOB access in @ref MeshTools::tipsify() that
     could trigger ASan or debug iterator errors
 -   With @ref Corrade/Utility/DebugStl.h not being included, @ref std::string

src/Magnum/Platform/GlfwApplication.cpp

@@ -145,13 +145,14 @@ Vector2 GlfwApplication::dpiScaling(const Configuration& configuration) {
     /** @todo */
     #endif
 
-    /* Use values from the configuration only if not overriden on command line.
-       In any case explicit scaling has a precedence before the policy. */
+    /* Use values from the configuration only if not overriden on command line
+       to something non-default. In any case explicit scaling has a precedence
+       before the policy. */
     Implementation::GlfwDpiScalingPolicy dpiScalingPolicy{};
     if(!_commandLineDpiScaling.isZero()) {
         Debug{verbose} << "Platform::GlfwApplication: user-defined DPI scaling" << _commandLineDpiScaling.x();
         return _commandLineDpiScaling;
-    } else if(UnsignedByte(_commandLineDpiScalingPolicy)) {
+    } else if(_commandLineDpiScalingPolicy != Implementation::GlfwDpiScalingPolicy::Default) {
         dpiScalingPolicy = _commandLineDpiScalingPolicy;
     } else if(!configuration.dpiScaling().isZero()) {
         Debug{verbose} << "Platform::GlfwApplication: app-defined DPI scaling" << _commandLineDpiScaling.x();

src/Magnum/Platform/Sdl2Application.cpp

@@ -206,13 +206,14 @@ Vector2 Sdl2Application::dpiScaling(const Configuration& configuration) {
     /* Handled below, warning printed only when using virtual DPI scaling */
     #endif
 
-    /* Use values from the configuration only if not overriden on command line.
-       In any case explicit scaling has a precedence before the policy. */
+    /* Use values from the configuration only if not overriden on command line
+       to something non-default. In any case explicit scaling has a precedence
+       before the policy. */
     Implementation::Sdl2DpiScalingPolicy dpiScalingPolicy{};
     if(!_commandLineDpiScaling.isZero()) {
         Debug{verbose} << "Platform::Sdl2Application: user-defined DPI scaling" << _commandLineDpiScaling.x();
         return _commandLineDpiScaling;
-    } else if(UnsignedByte(_commandLineDpiScalingPolicy)) {
+    } else if(_commandLineDpiScalingPolicy != Implementation::Sdl2DpiScalingPolicy::Default) {
         dpiScalingPolicy = _commandLineDpiScalingPolicy;
     } else if(!configuration.dpiScaling().isZero()) {
         Debug{verbose} << "Platform::Sdl2Application: app-defined DPI scaling" << _commandLineDpiScaling.x();

src/Magnum/Platform/Test/GlfwApplicationTest.cpp

@@ -25,10 +25,12 @@
 
 #include <Corrade/Containers/Optional.h>
 #include <Corrade/PluginManager/Manager.h>
+#include <Corrade/Utility/Arguments.h>
 #include <Corrade/Utility/DebugStl.h>
 #include <Corrade/Utility/Resource.h>
 
 #include "Magnum/ImageView.h"
+#include "Magnum/Math/ConfigurationValue.h"
 #include "Magnum/Platform/GlfwApplication.h"
 #include "Magnum/Trade/AbstractImporter.h"
 #include "Magnum/Trade/ImageData.h"
@@ -96,7 +98,17 @@ struct GlfwApplicationTest: Platform::Application {
     void drawEvent() override {}
 };
 
-GlfwApplicationTest::GlfwApplicationTest(const Arguments& arguments): Platform::Application{arguments, Configuration{}.setWindowFlags(Configuration::WindowFlag::Resizable)} {
+GlfwApplicationTest::GlfwApplicationTest(const Arguments& arguments): Platform::Application{arguments, NoCreate} {
+    Utility::Arguments args;
+    args.addOption("dpi-scaling").setHelp("dpi-scaling", "DPI scaled passed via Configuration instead of --magnum-dpi-scaling, to test app overrides")
+        .parse(arguments.argc, arguments.argv);
+
+    Configuration conf;
+    conf.setWindowFlags(Configuration::WindowFlag::Resizable);
+    if(!args.value("dpi-scaling").empty())
+        conf.setSize({800, 600}, args.value<Vector2>("dpi-scaling"));
+    create(conf);
+
     /* For testing resize events */
     Debug{} << "window size" << windowSize()
         #ifdef MAGNUM_TARGET_GL

src/Magnum/Platform/Test/Sdl2ApplicationTest.cpp

@@ -25,10 +25,12 @@
 
 #include <Corrade/Containers/Optional.h>
 #include <Corrade/PluginManager/Manager.h>
+#include <Corrade/Utility/Arguments.h>
 #include <Corrade/Utility/DebugStl.h>
 #include <Corrade/Utility/Resource.h>
 
 #include "Magnum/ImageView.h"
+#include "Magnum/Math/ConfigurationValue.h"
 #include "Magnum/Platform/Sdl2Application.h"
 #include "Magnum/Trade/AbstractImporter.h"
 #include "Magnum/Trade/ImageData.h"
@@ -141,7 +143,18 @@ struct Sdl2ApplicationTest: Platform::Application {
     #endif
 };
 
-Sdl2ApplicationTest::Sdl2ApplicationTest(const Arguments& arguments): Platform::Application{arguments, Configuration{}.setWindowFlags(Configuration::WindowFlag::Resizable)} {
+Sdl2ApplicationTest::Sdl2ApplicationTest(const Arguments& arguments): Platform::Application{arguments, NoCreate} {
+    Utility::Arguments args;
+    args.addOption("dpi-scaling").setHelp("dpi-scaling", "DPI scaled passed via Configuration instead of --magnum-dpi-scaling, to test app overrides")
+        .addSkippedPrefix("magnum", "engine-specific options")
+        .parse(arguments.argc, arguments.argv);
+
+    Configuration conf;
+    conf.setWindowFlags(Configuration::WindowFlag::Resizable);
+    if(!args.value("dpi-scaling").empty())
+        conf.setSize({800, 600}, args.value<Vector2>("dpi-scaling"));
+    create(conf);
+
     /* For testing resize events */
     Debug{} << "window size" << windowSize()
         #ifdef MAGNUM_TARGET_GL