SkLightingShader normal vector CPU computation refactor.

SkLightingShader normal vector CPU computation refactor.

The purpose of this change is to refactor the handling of normal maps out of SkLightingShader, laying the groundwork to eventually allow for multiple normal sources.

This CL's base was the CL for GPU handling: https://codereview.chromium.org/2043393002/

What this CL includes:

- A refactor of the SkLightingShader context's code that deals with reading normals off of a normal map. This is now abstracted out into a NormalSource::Provider class that the context uses.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2050773002

Review-Url: https://codereview.chromium.org/2050773002