glWaitSemaphoreEXT and glSignalSemaphoreEXT functions optionally request
buffer and image barriers to be performed by the implementation.

If any barriers are present, a single global memory barrier is inserted
to take care of memory accesses.

In both functions, buffer and image memory barriers are used to perform
queue ownership transfers to ANGLE's queue (glWaitSemaphoreEXT) or the
EXTERNAL queue (glSignalSemaphoreEXT).

In glWaitSemaphoreEXT, the given layouts are information regarding how
the external entity (the caller) has modified the images' layouts, and
is used to update ANGLE's internal state tracking.

Bug: angleproject:3289
Bug: 1026673
Change-Id: Ic478a8813df727c89413c8ae2adf42b5c1d06069
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1933016
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>