DXT1 has a specific 'BLACK' code that results in transparent black pixels when
sampled. D3D does not have specific RGB-only DXT1 formats like OpenGL does so
when this code is encountered, we sample 0 alpha for these pixels when GL would
expect 1 because the alpha channel should not exist.

Work around this by converting to DXT3 RGBA, adding an extra block of 1.0 alpha
pixels for each color block.

Mac Intel OpenGL requires additional workarounds to always sample 1.0 alpha.
Set the texture swizzle parameters to force it.

BUG=angleproject:3729

Change-Id: Ia3647085acd97bb01af4e95ef3f6f21dcfb6a554
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1804880
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Ian Elliott <ianelliott@google.com>