Even though this is the "OpenGL" surface implementation, this can still happen
in e.g. ddraw if no swapchain is created yet. That's something we should fix,
but not today.

At the moment this is only marginally useful, since it just avoids silly
things like color keyed depth fills that are probably invalid anyway. However,
the idea is to gradually move normal color fill and blit handling out of
IWineD3DSurfaceImpl_BltOverride() and surface_cpu_blt(), and eventually make
those functions go away completely.

wined3d: IWineD3DSurfaceImpl_BltOverride() is always passed valid source and destination rectangles.

So no need to call surface_get_rect() anymore.