Add SkLights class

The SkLights class is intended to be passed to the drawLitAtlas call (and then on to SkLightingShader).

This CL is calved off of https://codereview.chromium.org/1261433009/ (Add SkCanvas::drawLitAtlas call).
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Review-Url: https://codereview.chromium.org/1255133004
2 files changed