Add fast normalize for SkLightingImageFilter.
Add fast normalize for SkLightingImageFilter.

The normalize routine in SkPoint3 is very robust. However, for simple
lighting cases we prefer speed over robustness. This fixes a perf
regression in smoothness.tough_filters_cases.

BUG=chromium:510562

Review URL: https://codereview.chromium.org/1240023002

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