When doing an EXT_copy_image copy, renderbuffers
were being recreated due to a mismatch in sample
count values.

Bug: angleproject:3593
Test: dEQP-GLES31.functional.copy_image.*renderbuffer*
Change-Id: Ic6b85ea098ab6382b56520b1ca6b76ac25801b54
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2538076Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>