Correctly generate mipmaps for sRGB textures. 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>