We would often pad incorrectly given the constraints of the max stride.
We shouldn't really be rounding up the buffer size, but we should
instead be adding the max alignment size to the end of the buffer.

Bug: angleproject:4428
Change-Id: Id2afc572c85985548a18f60b42cdc388d83d5c4c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2071235
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>