fix SkJumperHSL blend modes

I took a new, unprincipled approach here, which is to just mimic the
legacy code path exactly (e.g. hue_modeproc in SkXfermode.cpp).

This fixes how we handle alpha in these blend modes, and ought to be
faster by avoiding the unpremul.

BUG=skia:6621

Change-Id: I21635290985ff42d9421d2718f7a88cf44a85d7f
Reviewed-on: https://skia-review.googlesource.com/16711
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed