Pack: row length, skip rows, skip pixels.
Unpack: image height, skip images, skip rows, skip pixels.

BUG=angleproject:512
BUG=angleproject:1095

Change-Id: I11e3bc05d23419b72c92b96aabd3f0bacd983626
Reviewed-on: https://chromium-review.googlesource.com/304370
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>