GL: Support Rectangle textures as sources for glCopyTextureCHROMIUM. 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/+/1726849 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>