sRGB textures need to linearize their data before averaging, this worked
correctly on the GPU mipmap generation functions but not the CPU ones.

BUG=angleproject:2019

TEST=conformance/extensions/ext-sRGB

Change-Id: I554dac89b12acf5ebf5b7cab6f6faf0bce5168a6
Reviewed-on: https://chromium-review.googlesource.com/506241
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>