remove burst mode in SkRPBlitter

This shader burst mode is only implemented by linear gradient,
and it doesn't seem to handle color management the way we expect.

This CL should be enough alone to fix the attached bug.  As a follow up
we can now remove shadeSpan4f(), which has one or two more incidental
call sites.

Bug: skia:8730
Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Change-Id: I8fe5fab5e003702106846dcad92505bf1c70fba3
Reviewed-on: https://skia-review.googlesource.com/c/188030
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
3 files changed