Add support for intrinsic step() in RP codegen.

This is implemented roughly as `float(lessThan(edge, x))`.

Change-Id: Iff2698cd97e50acb5f0287bc6e68e453165f2a8b
Bug: skia:13676
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/628156
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
2 files changed