Context's staging buffer is shared for all kind of usages. We should
pass in alignment requirement for allocate call whenever it is different
from the default alignment. This adds allocateWithAlignment call to
DynamicBuffer call and switch ImgeHelper's usage of it to
allocateWithAlignment

Bug: b/164511310
Change-Id: Idcb6b6f95d6862ee6cb8fca9c164910b7e085a17
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2373590
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>