Changes since the v4:
  - Avoid having two mf_init function declared (missing #if !HAVE_UWP)
  - Better commit message about linking with UWP

Changes since the v3:
   - Library handle and function pointer are no longer in MFContext.
   - If UWP is enabled, avcodec will be linked directly against MFPlat.DLL.
   - MediaFoundation functions are now called like MFTEnumEx, like Martin
     Storsjö suggested in his review of the v3.

I forgot to mention it on earlier versions, this patch addresses
https://trac.ffmpeg.org/ticket/9788.

// Trystan