Support hardware RGB565 textures with DX11

[ZivDero]

As far as I can see, DirectX11 (and probably 12, too), perfectly support RGB565 textures via DXGI_FORMAT_B5G6R5_UNORM.
Adding

    case SDL_PIXELFORMAT_RGB565:
        return DXGI_FORMAT_B5G6R5_UNORM;

to SDLPixelFormatToDXGITextureFormat and skipping over the IsSupportedFormat call in SDL_CreateTextureWithProperties was sufficient to get a working texture that gave me a ~3x perf boost due to offloading color conversion to the GPU.

[ZivDero]

Update: DX12 seems to require soe extra adjustments