We accumulate image barriers in two places:

 * for GL sampler textures
 * for GL framebuffer attachments (Render Targets)

Then we issue the barriers together in a single call before the RP.
This fixes a bug where we were missing a layout transition in some
cases when transitioning between a sampler and a render target. It
should also be faster to issue a single barrier before a RP than issue
several smaller barriers.

Bug: angleproject:3539
Bug: angleproject:4029
Change-Id: I180b770f0df6b44d209e5c618ba36bcc1c6372e4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2044236
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>