Fixed DX SetRenderTarget Bug

This fix ensures that a render target cannot be bound as a render target and as a texture at the same time.

This is disallowed by DirectX and when you manage to do it would cause the render target apply to fail.  I don't think this same issue can occur on OpenGL because of how the API works, so I limited the fix to DirectX only until someone reports otherwise.

See #1868 for a sample that causes this to occur.