Add kSkBlendModeCount

I keep bumping into the need for this

Change-Id: I69384f7d590c163fd6244bdc64cc5e48450fecd4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/540171
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
8 files changed