Always use raster pipeline for SkSweepGradient

Always use raster pipeline for SkSweepGradient

Add an opt-in mechanism (SkShader::isRasterPipelineOnly) and switch
SkSweepGradient to always-RP.

This also repurposes the existing SK_LEGACY_SWEEP_GRADIENT guard.

Change-Id: Iabf0a701eda56f009d331120f0c987f96bd35d27
Reviewed-on: https://skia-review.googlesource.com/16912
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>