fixed warnings in gstmfxallocator_d3d11.c
fixed warnings in gstmfxallocator_d3d11.c

VS2015 compiler wasn't happy we use ID3D11Texture2D and ID3D11Resource
interchangeably, so now we cast it explicitely

ph0b/gstreamer-media-SDK,ishmael1985/gstreamer-media-SDK