This gives us the flexibility to use copy internally for Textures
as well as FBO attachments. This will be useful for the TexSubImage
performance workaround.

BUG=angle:729

Change-Id: I4df8ef3a5a928d44ef84100cd96a5d35f12b47fa
Reviewed-on: https://chromium-review.googlesource.com/219863Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>