draw vertices: put SkVM implementation behind a flag

Change-Id: I8bcaad6169d5e243b48984657347efed7063b654
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/433496
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Reed <reed@google.com>
1 file changed