Add an SkVM panel to viewer's debug UI

Caveats:
  - You want to run viewer with `--skvm` to see much/anything
  - The cache is LRU, and doesn't get flushed automatically. Hitting
    'Clear' will flush it, so it will pick up only the blitters used
    on the current slide. (Otherwise they accumulate as you navigate).
  - No way to determine which blitter is for which primitive, yet.
    I'd like to do the GPU-style magenta highlight, but that may be
    tricky, because we need to preserve the semantics of the original
    blitter (including destination color type, most importantly).

Change-Id: I2df763fdb697d87471ca0816a3b7087ffb4fc4e4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/467783
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed