Depth pitch computations were not taking into account the block size
and simply multiplying the row pitch with the pixel height.  This caused
our load functions to use a very high depth pitch, reading past the end
of the user-supplied buffer.

BUG=angleproject:3190
BUG=angleproject:3920

Change-Id: I4ef4763b542735993568c51ae4b5a235659b9094
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1811837Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Ian Elliott <ianelliott@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>