It's possible to destroy an SDL_Renderer without freeing it using
SDL_DestroyRendererWithoutFreeing(), which is used to make it possible
to destroy windows and their renderers in either order. However, if a
renderer has already been destroyed before it is freed (e.g., the window
was destroyed before the renderer), the object is never marked invalid.
This means the SDL_Renderer is reported as leaked, even if
SDL_DestroyRenderer() is called.
SDL_GetWindowSurface() will trigger this, as the window texture is
cleaned up _after_ the window destroys its associated renderer. This
makes it impossible to use SDL_FRAMEBUFFER_ACCELERATION without
triggering a leak warning.
Fix this by unconditionally marking the SDL_Renderer object as invalid
in SDL_DestroyRenderer().
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