Gradient shaders: make fPtsToUnit const, pre-cache getType().

Gradient shaders: make fPtsToUnit const, pre-cache getType().

This prevents races when calling fPtsToUnit.getType() from multiple threads.

This introduces a small amount of redundant code in SkTwoPointRadialGradient,
but it's probably optimized together into no extra run-time work.

BUG=437511

Review URL: https://codereview.chromium.org/793763003