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Refactors and cleans-up code, transforming GLHelperScaling's internal
"ScalerImpl" class into a GLScaler::ScalerStage class. A ScalerStage
chains together with other ScalerStages to perform each step from the
source input to ultimate scaled output. (A future change will provide
the logic for deciding how to create/configure ScalerStages and chain
them together.)

A major change other than the refactoring: The math that calculates each
shader's "overscan" padding has been revisited and corrected. New math
is included in this change, along with pixel tests that confirm it is
correct (i.e., tests the math, but also runs the shader programs to
prove the numbers jive).

Bug: 870036
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I3dbbbb8f8cca45006895faed6d7501ceab5cc1cb
Reviewed-on: https://chromium-review.googlesource.com/c/1285328Reviewed-by: Xiangjun Zhang <xjz@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601341}