Fix some errors in the GL backend related to source and destination mipmap
targets.

BUG=angleproject:1356

Change-Id: I030529c8626f3bc30dbb4f7f859a02ba56e315e3
Reviewed-on: https://chromium-review.googlesource.com/527653Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>