Move sRGB <-> linear conversion components to their own files.

Move sRGB <-> linear conversion components to their own files.

This makes them a little easier to use outside SkColorXform code.

I've added some notes about how best to use them and their eccentricities, and added a test.

Ultimately any software sRGB <-> linear conversion should funnel somehow through here.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2128893002
CQ_EXTRA_TRYBOTS=client.skia.android:Test-Android-GCC-Nexus5-CPU-NEON-Arm7-Release-Trybot;client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Committed: https://skia.googlesource.com/skia/+/45e58c8807179638980aae8503573b950b844e4c
Review-Url: https://codereview.chromium.org/2128893002