GitLab

Adds the implementation that determines which shader programs to use,
and in what order, to execute scaling and format conversion. Also, adds
pixel tests to check all the various features provided by GLScaler (via
its Parameters).

Bug: 870036
Change-Id: I7cb1d64d6df79a3a6221c2ab6cdf4b504b26c5b1
Reviewed-on: https://chromium-review.googlesource.com/c/1298737
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Xiangjun Zhang <xjz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603762}