The multiply/add uniforms are to mask the output of the shader based
on the destination format.  The pre/unmuliply logic should be performed
on the source data without masking applied.

TEST=conformance/textures/canvas/tex-2d-rgb-rgb-unsigned_byte.html

BUG=angleproject:2192

Change-Id: I6e5f29a7d56e18bd1a81ff56500edb4c6b55eb55
Reviewed-on: https://chromium-review.googlesource.com/730494
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>