Chrome uses rectangle textures on mac to copy data.  ANGLE's frontend allowed
them as CopyTexture sources but did not implement them in the backend.

TEST=Any WebGL test on Mac

BUG=982294

Change-Id: If2e40292b22c4f49676e3ece8cc6fa126c5b7b94
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1726849Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>