Update index of contour start point correctly in addPath

The index was computed incorrectly when adding a closed path
to a non-empty path.

Also restores clause incorrectly removed from a test condition,
updates tests to detect this failure, and clarifies documentation.

Bug: https://g-issues.skia.org/issues/40043389
Change-Id: Ia1106496c3e6a52f4a607317c3185066d4e6160c
Fiddle: https://fiddle.skia.org/c/ad174f28b0081cfa6129fad2863fb426
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/436476
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
3 files changed