glReadPixels uses a temporary buffer for readback and was missing an
onBufferTransferWrite() call.

Bug: angleproject:5669
Change-Id: I3c6daba176e332711fa1ab51e279526717486dd0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2717005Reviewed-by: Jie A Chen <jie.a.chen@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>