Previously, we wouldn't update the dirty bits when switching from a
workarounded alpha/alpha/uint8 (r8/red/uint8) to something else.

Found via TextureUploadFormatTest.

BUG=angleproject:2232
Change-Id: Idba63282f8a65daec5675798d3516345ca941447
Reviewed-on: https://chromium-review.googlesource.com/756641
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>