There is no such thing as image2DRect in GLSL.  imageRect (the right
type) is present, but the code was mistakenly using image2DRect.

Bug: angleproject:4889
Change-Id: I42bacd2ad8866c715af0d9711bebc95897607354
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2878095Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>