Add new op for merging the condition mask with the stack-top.

This will enable us to implement basic conditionals like if
statements, and ternaries.

Change-Id: Id331e277ba6fee2fcc322a6a42117bc066778278
Bug: skia:13676
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/611957
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
3 files changed