Make tessellation join type values negative

This feels more intuitive and it allows the override number of
segments to be positive.

Bug: skia:10419
Change-Id: I0a57d9b114abb4ec8472e1c405ace4385be23e42
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/306417
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
3 files changed