This is preparation for crrev.com/c/2462604. We run out of bits in the
struct PackedAttachmentOpsDesc. This expands it from 16 bits to 32 bits
(two uint16_t), gives us a lot more room for current and future needs.

Bug: angleproject:5055
Change-Id: I492cbd18c3b71b76b5b48652094409e76c5721b1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2465615Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>