Use the 4f impl for legacy/8888 linear gradients

This is a partial revert of https://skia-review.googlesource.com/c/20280,
adding back L32 support for 4f gradients and switching off the legacy
impl.

If it sticks, we should be able to completely delete the legacy gradient
code.

Change-Id: Iaa6d722ea4dfd8e4d959bb76eca815a30ca7098f
Reviewed-on: https://skia-review.googlesource.com/31425
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
6 files changed