Extend arithmode_blender GM slide.
Extend arithmode_blender GM slide.

Previously, we had two panels drawn differently that were meant to be
visually identical:
- Draw one image normally, then draw a second image on top of the
  first with a blender enabled.
- Draw both images in one paint, using SkImageFilters::Blend to
  apply the arithmetic SkBlender.

Now, I've added a third approach:
- Paint a runtime effect which samples from both images, and blends
  between them by evaluating the arithmetic SkBlender.

We have a surprising lack of GMs which evaluate runtime blends in the
shader/color filter stage (as opposed to during the final blend).

Change-Id: I69f2d5584db4e00fd6335e353943833a0ea7d2d0
Bug: b/238755735
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/581596
Commit-Queue: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>

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