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)