The previous logic of only checking the currently enabled draw buffers was
incorrect.  Also updated the logic to be more lenient and allow multiple
slices or mips of the same texture.

BUG=483282

Change-Id: I0d406d1a9a96e48342baefbaf11e0c2ecc6d390c
Reviewed-on: https://chromium-review.googlesource.com/361533Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>