SkSLSlide: Add shadertoy iResolution & iMouse

Verified behavior with: https://www.shadertoy.com/view/Mss3zH

Change-Id: Ic5fe8ab0488a3c6917e5f46ccd376867fc868c78
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/343516
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed