ui/sdl: add QEMU_ENABLE_SDL_LOGGING setting/environment

Enable SDL logging when QEMU_ENABLE_SDL_LOGGING variable is set, as
suggested by Sam Lantinga, upstream SDL maintainer.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

ui/sdl2.c

@@ -843,6 +843,10 @@ static void sdl2_display_init(DisplayState *ds, DisplayOptions *o)
     }
 #endif
 
+    if (SDL_GetHintBoolean("QEMU_ENABLE_SDL_LOGGING", SDL_FALSE)) {
+        SDL_LogSetAllPriority(SDL_LOG_PRIORITY_VERBOSE);
+    }
+
     if (SDL_Init(SDL_INIT_VIDEO)) {
         fprintf(stderr, "Could not initialize SDL(%s) - exiting\n",
                 SDL_GetError());