For some BGRA-backed formats (RGBA4, R5G6B5, RGB5A1), our ReadPixels
implementation wasn't aware the BGRA format didn't exactly match the
RGBA format. For these it would do the 'fast path' memcpy method, when
it should stop and do the slow pixel-by-pixel packing method.

BUG=angleproject:1407
BUG=chromium:616176

Change-Id: Ie24758513af6f9ef87f0aa503135456c96493701
Reviewed-on: https://chromium-review.googlesource.com/352252Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>