Uses CPU copy if the buffer is CPU visible. Also add dirty bit check to avoid repeated conversion on clean data

Bug: angleproject:4400
Change-Id: I1f472703ebc4a65ee0de129c27f56b081748f900
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2057758Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tobin Ehlis <tobine@google.com>
Commit-Queue: Tobin Ehlis <tobine@google.com>