Emulated alpha framebuffers should never modify the alpha channel. Our
blitting code didn't take this property into account.

TEST=conformance2/rendering/blitframebuffer-resolve-to-back-buffer.html

BUG=angleproject:4645

Change-Id: I1797934df7e7aa520bacfba8a379f6545e38753e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2210023Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: James Darpinian <jdarpinian@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>