In preparation for adding support for more YUV formats
add an 'isYUV' field that tracks whether a format is a
YUV format. For now it is set if the format string contains
the substring 'PLANE'. This is not the case for all known YUV
formats but the condition can be expanded when required.

Bug: angleproject:5773
Change-Id: Icb574037dc8105584931835dab0b60e8bafa3262
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2774762Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>