Add support for VP8/VP9 codecs (usually via media-libs/libvpx)

Packages describing “vpx” as local USE flag

Package“vpx” Flag Description
media-tv/tvheadendEnable transcoding with the VP8/VP9 video codecs
media-video/ffmpegEnables VP8 and VP9 codec support using libvpx: Decoding does not require this to be enabled but libvpx can also be used for decoding; encoding requires this useflag to be enabled though.

All packages providing a “vpx” USE flag (13)