Relocate the specialized linear gradient interval builder

The specialized interval setup works really well for (4f) linear
gradients, but it seems unlikely to benefit other gradient subclasses.

Since it gets in the way of a general/fallback gradient impl, let's move
this code to Sk4fLinearGradient.

R=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1764183002

Review URL: https://codereview.chromium.org/1764183002
4 files changed