3D subregions were not properly supported in Image::copyTo()

Bug: b/144353295
Change-Id: I445881b37be310401b38cffcb6003a1e4461bed8
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38471
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>