Fix input lag in SDL2 backend

If you spam keyboard inputs, they'll eventually lag behind. This was
caused by improperly porting the WinAPI logic to SDL2 and CSE2's
fancy backend-abstraction system.

src/Backends/Platform/SDL2.cpp

@@ -165,7 +165,7 @@ void PlaybackBackend_EnableDragAndDrop(void)
 
 bool Backend_SystemTask(bool active)
 {
-	if (SDL_PollEvent(NULL) || !active)
+	while (SDL_PollEvent(NULL))
 	{
 		SDL_Event event;