Tiling support for SkSweepGradient

Expand the sweep gradient definition to include a color stop angular range ([0, 360] by default).

Color stop positions in [0,1] are mapped to this range, and drawing outside is controlled by a
tile mode param.

This is closer to the CSS gradients spec and allows us to use fewer color stops in Blink conic
gradients.

Impl-wise, the remapping is effected after t calculation, and before tiling.

Change-Id: I5d71be01d134404d6eb9d7e2a904ec636b39f855
Reviewed-on: https://skia-review.googlesource.com/27704
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
8 files changed