mft: improve decoding and clean code
- Use the new HXXX helper API to send the PPS/SPS/VPS to the decoder.
- Add more logs
- Implement drain and flush for synchronous decoders (the ones always used)
- Prepare the code for D3D11 output
- use MFStartup/MFShutdown as advised in the doc
- Don't load the mfplat DLL manually, we already link with it (works even in UWP)