Add NV12 texture conversion support.

If textures 1 and 2 passed into MakeFromYUVTexturesCopy are the same,
then treat the input as NV12 and sample from the g component of texture
2.

BUG=skia:5209
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2016593002

Review-Url: https://codereview.chromium.org/2016593002
6 files changed