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.

Re-land with fix for test expectations.

BUG=angleproject:512
BUG=angleproject:1095

Change-Id: I71d8d3bd8fc1f2c75ca16ac2634d5eafcbd71f26
Reviewed-on: https://chromium-review.googlesource.com/305522Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>