Will allow us to determine if we're clearing the read or draw FBO. Then
we can stash clears for the draw FBO only and issue them immediately
for the read FBO in the Vulkan back-end.

Bug: angleproject:4517
Change-Id: Ifc043317d6156a75749b13f9d2c44a17e14ee378
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2139997Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>