Added SDL_GetTextureDXGIResource() to get the DXGI resource associated with a render texture.

Also switched the D3D11 and D3D12 renderers to use real NV12 textures for NV12 data.

The combination of these two changes allows us to implement 0-copy video decode and playback for D3D11 in testffmpeg without any access to the renderer internals.

src/render/SDL_sysrender.h

@@ -212,6 +212,8 @@ struct SDL_Renderer
     void *(*GetMetalLayer)(SDL_Renderer *renderer);
     void *(*GetMetalCommandEncoder)(SDL_Renderer *renderer);
 
+    IDXGIResource *(*GetTextureDXGIResource)(SDL_Texture *texture);
+
     /* The current renderer info */
     SDL_RendererInfo info;