Improve the skvm debug panel

Use SkVMBlitter::DebugName for the string. This is easier to read, and
has some actually useful information about the blitter.

Added magenta highlight of the hovered element, similar to the GPU.

Change-Id: Ic9b5a0f61e092c8aa555f375d5d2f2de22cc45fe
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/467977
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed