Platform: request forward-compatible core context in Sdl2Application.

Should avoid more potential issues when the driver has to return older
version because the newer has some features removed.

src/Magnum/Platform/Sdl2Application.cpp

@@ -145,6 +145,7 @@ bool Sdl2Application::tryCreateContext(const Configuration& configuration) {
         SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 1);
         #endif
         SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
+        SDL_GL_SetAttribute(SDL_GL_CONTEXT_FLAGS, SDL_GL_CONTEXT_FORWARD_COMPATIBLE_FLAG);
         #else
         /* For ES the major context version is compile-time constant */
         #ifdef MAGNUM_TARGET_GLES3
@@ -196,6 +197,7 @@ bool Sdl2Application::tryCreateContext(const Configuration& configuration) {
         SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 2);
         SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 1);
         SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_COMPATIBILITY);
+        SDL_GL_SetAttribute(SDL_GL_CONTEXT_FLAGS, 0);
 
         if(!(_window = SDL_CreateWindow(configuration.title().data(),
             SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,