Update SkLightingShader to support rotation

This also:
makes the SkLightingShader handle normal maps where the rects aren't aligned between the diffuse and normal maps.

adds a light aggregating class (Lights) to SkLightingShader (along with a Builder nested class).

Split out of https://codereview.chromium.org/1261433009/ (Add SkCanvas::drawLitAtlas call)

Committed: https://skia.googlesource.com/skia/+/45b59ed6e4e231814dbdb9f707b3d2a7ee50de84

Review URL: https://codereview.chromium.org/1291783003
17 files changed