Add a GN flag 'skia_enable_skvm'.

This flag currently defaults to true. When it is set to false, SkRP is
enabled instead, and the SkVMBlitter class becomes a no-op. (Make
always returns null; blit does nothing.)

Change-Id: Icda9be4e1c917aed73d45f49c38e5b0518726074
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/674336
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
6 files changed