The input coordinates of blitResolveStencil compute shader start
from 0,we need to align the sample position to start from 0 if x/y
is flipped.

Test: dEQP-GLES3.functional.fbo.invalidate.*.unbind_blit_msaa_*
Bug: b/159995959
Change-Id: If0c9f5b7cacddbe1a2d7062469a757a63bcc1378
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2392162
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>