Implement support for YV12 texture sampling.

Bug 21572252

Change-Id: Iaf54b4d960dbc243c40f981e1c73c199481e2d28
Reviewed-on: https://swiftshader-review.googlesource.com/3930
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
12 files changed