There is a good deal of overlap in the way these shaders calculate
offsets, flip etc.  This change merges the two shaders together, as well
as the UtilsVk functions that invoke them.

This is in preparation for adding blit functionality to the same
shader.

Bug: angleproject:3200
Change-Id: Id7c2cf12e05db1802ae497766fb361fe4bef6479
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1635750
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>