Add GM slide demonstrating runtime evaluation of null runtime effects.
Add GM slide demonstrating runtime evaluation of null runtime effects.

This slide should display three matching purple swatches.
The swatches demonstrate evaluating a null shader, color filter, and
blender.

Change-Id: Ia75847fa20f96595c112e0fad05ac6bebf2fc7d7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/580014
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia