D3D11: Added code to handle GPU-device-removed scenarios

These scenarios can happen when a GPU is switched, its driver updated, or in
some virtual machines (such as Parallels) are suspended and then resumed.  In
these cases, all GPU resources will already be lost, and it's up to the app to
recover.

For now, SDL's D3D11 renderer will handle this by freeing all GPU resources,
including all textures, and then sending a SDL_RENDER_TARGETS_RESET event.
It's currently up to an app to intercept this event, destroy all of its
textures, then recreate them from scratch.