This is prerequisite work to dynamic uniform buffer updates.

1- Alignment parameter to be able to allocate by chunks of this
alignment.
2- Out boolean to indicate if a new VkBuffer has been assigned
on the allocate operation.

Bug:angleproject:2392

Change-Id: If346e13200455febbe78045e908ae89c9dc93cdb
Reviewed-on: https://chromium-review.googlesource.com/965612
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>