Final attempt to fix shaders w/o version directive.

Took me insane amount of time. I blame those 12-hour working days.

src/Magnum/Shader.cpp

@@ -786,13 +786,15 @@ Shader& Shader::addSource(std::string source) {
 
         /* Fix line numbers, so line 41 of third added file is marked as 3(41)
            in case shader version was not Version::None, because then source 0
-           is the #version directive added in constructur.
+           is the #version directive added in constructor.
 
            If version was Version::None, line 41 of third added file is marked
            as 2(41). We apparently can't add even the #line directive before
            the potential `#version` directive -- in that case the first source
-           file is not marked with any file number, but that's equivalent to
-           the default where first file is marked as 0. */
+           file is not marked with any file number, thus having number 0. In
+           order to avoid complex logic in compile() where we assert for at
+           least some user-provided source, an empty string is added here
+           instead. */
         if(!_sources.empty()) _sources.push_back("#line 1 " +
             #if !defined(CORRADE_TARGET_NACL_NEWLIB) && !defined(CORRADE_TARGET_ANDROID)
             std::to_string((_sources.size()+1)/2) +
@@ -800,6 +802,8 @@ Shader& Shader::addSource(std::string source) {
             converter.str() +
             #endif
             '\n');
+        else _sources.emplace_back();
+
         _sources.push_back(std::move(source));
     }
 

src/Magnum/Shader.h

@@ -595,7 +595,9 @@ class MAGNUM_EXPORT Shader: public AbstractObject {
          *
          * Adds given source to source list, preceeded with @c \#line directive
          * marking first line of the source as `n(1)` where n is number of
-         * added source. If passed string is empty, the function does nothing.
+         * added source. The source number `0` is @c \#version directive added
+         * in constructor, if any. If passed string is empty, the function does
+         * nothing.
          * @see @ref addFile()
          */
         Shader& addSource(std::string source);

src/Magnum/Test/ShaderGLTest.cpp

@@ -200,6 +200,7 @@ void ShaderGLTest::addSourceNoVersion() {
 
     #ifndef MAGNUM_TARGET_GLES
     CORRADE_COMPARE(shader.sources(), (std::vector<std::string>{
+        "",
         "#version 120\n",
         "#line 1 1\n",
         "#define FOO BAR\n",
@@ -208,6 +209,7 @@ void ShaderGLTest::addSourceNoVersion() {
     }));
     #else
     CORRADE_COMPARE(shader.sources(), (std::vector<std::string>{
+        "",
         "#version 100\n",
         "#line 1 1\n",
         "#define FOO BAR\n",