This was stomping all over memory beyond the image.

Bug: b/nnn
Test: dEQP-VK.pipeline.render_to_image.core.*
Change-Id: Ie8738da787ea98c0d6431970cf6301bd6b5b5323
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/30528Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>