This extension allows users of ANGLE to make contexts current with surfaces
that were created with different bit depths.

BUG=angleproject:1223

Change-Id: I9072c256e0a36aee1ce41ab69cb9a04240bf0521
Reviewed-on: https://chromium-review.googlesource.com/314935Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tryjob-Request: John Bauman <jbauman@chromium.org>
Tested-by: John Bauman <jbauman@chromium.org>