SDL2: Explicitly use RGB8 color buffer.

src/citra/emu_window/emu_window_sdl2.cpp

@@ -73,6 +73,10 @@ EmuWindow_SDL2::EmuWindow_SDL2() {
     SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 3);
     SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
     SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
+    SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 8);
+    SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 8);
+    SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 8);
+    SDL_GL_SetAttribute(SDL_GL_ALPHA_SIZE, 0);
 
     std::string window_title = Common::StringFromFormat("Citra | %s-%s", Common::g_scm_branch, Common::g_scm_desc);
     render_window = SDL_CreateWindow(window_title.c_str(),