libvpx only emits:
VPX_IMG_FMT_{I420,I422,I440,I444,I42016,I42216,I44016,I44416}
and additionally supports YV12 as input.

interleaved yuv, rgb and alpha formats are unused.

Change-Id: Ie2ab1099e950c6e696f475d46882f5c47a174042