Pack: row length, skip rows, skip pixels.
Unpack: image height, skip images, skip rows, skip pixels.
Note that PBOs are not covered by this change.

BUG=angleproject:512
BUG=angleproject:1095

Change-Id: Ia2fd7e52615d4aa08011dd615fcc20b79672d355
Reviewed-on: https://chromium-review.googlesource.com/304908Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>