Refactor ComputeFirstDirection

Earlier patch sets were more aggressive, and removed the first
direction enum entirely. I think that is the right way to go:

- remove the firstdirection enum entirely
- don't try caching it in the path

but the change was a little tricky to do all at once, so I
propose just landing this partial change first.

Change-Id: Ic06a7fe9ee84a021da65616f2f4117d180df1440
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1049076
Reviewed-by: Florin Malita <fmalita@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Mike Reed <mike@reedtribe.org>