Generic 4f gradient T sampler fallback
Add a generic T sampler fallback impl which uses T series produced by
subclasses mapTs() overrides. The fallback path uses the same interval
structures as the current optimized linear4f impl, but always sorted
in stop order (never inverted to match dx/increasing x order).
Enable the new mechanism for 4f linear w/ perspective.
Other boring changes:
* relocate the interval builder (back) to the base class
* add a private header for shared templates
BUG=skia:
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Review URL: https://codereview.chromium.org/1783823002
7 files changed