Adds the implementation to support gamma-aware scaling: It uses half-
floats to increase the precision of the color values, and adds an
"import stage" to linearize the color values before any texture sampling
or scaling occurs.

Added a pixel test to prove the gamma-aware versus non-gamma-aware
configuration of GLScaler produces the expected results.

Bug: 870036, 810131
Change-Id: Ie68121add789169dbda0a578f03a257d089d4f6c
Reviewed-on: https://chromium-review.googlesource.com/c/1297540
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Xiangjun Zhang <xjz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603797}