Bug 1067377 - Use VPX_ prefixed defines in VP8TrackEncoder. r=giles

The unprefixed IMG_FMT and PLANE defines from libvpx 0.9.x
are disabled by default subsequent to the 1.3.0 release.
Bug 1067377 - Use VPX_ prefixed defines in webrtc. r=rjesup
Bug 1067377 - Use VPX_ prefixed defines in WebMReader. r=kinetik