Fix the formula for fNumRadialSegmentsPerRadian in GrStrokePatchBuilder

GrStrokePatchBuilder had the wrong formula. This CL moves the formula to
GrStrokeTessellateOp and calculates it in one spot for the builder and
tessellator both. It also fixes a bug that was hiding behind this
formula error and updates some variable names.

Bug: skia:10419
Change-Id: I908d3960b16cac8dca0d40ff4116a3f5c5beed06
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/328416
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
7 files changed