Add separate src and dst arguments to runtime-blend SkSL functions.

When called in the typical FP context, we only have a paint color, not a
separate source and destination color. So we pass the paint color in
each slot. This seems counterintuitive, but the upshot is that a runtime
blender will now pass the paint color as the input color to the source
effect and dest effect. (In a followup CL, sampling directly from a
runtime blender will allow distinct values to be passed into each slot,
and these values will be honored as-is.)

Change-Id: I96b016527d1066f0431a763f6609c3983b4854ec
Bug: skia:12257
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/433516
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
4 files changed