Remove dead gradient perspective code

The cool bit here is that Sk4fLinearGradient is now always fast, and never calls the slow
base-class impl.  This means we can rip out Sk4fGradientBase::{mapTs, shadeSpan4f, etc}.

Change-Id: Id3788bc810873b2a209f66efa4187c84b3397e2f
Reviewed-on: https://skia-review.googlesource.com/22366
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
10 files changed