The logic to calculate the image binding offset for each element of an
array of array of images was incorrect, in that it used the arraySize
accumulated so far to offset the next image's binding.

Bug: angleproject:5535
Change-Id: I2dc3ce7bbf7b77302d1b56b4701bec2e990fdae1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2635452Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>