contrib: bump ffmpeg

A non exhautive list of what this bump brings:
- Direct 3D 11 hardware decoding (UWP)
- Uniform API for vaapi, vdpau, videotoolbox, dxva2 and d3d11va
- 32 bit MediaCodec fix (Android hardware decoding)
- Non X screen capture on Linux (Wayland support)
- Better VP9 support
- Various bug fixes and optimizations

Due to problems with the VideoToolbox acceleration, the bump for MacOS
is postponed.

Change-Id: Id05a1d789c90ea1321f10e801a036c47b6f76d88