Implements more pack/unpack states. 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>