Remove ambient clamp hack for analytic shadows.

The hack was added to make analytic ambient shadows match the geometric
ones exactly. Removing it does produce a slight difference, but it's
only visible at full black values (and then, only slightly) and this
makes room to pass a blend parameter for falloff.

Also fixes an issue with filling out arcs in the shadow tesselator.

Bug: skia:7486
Change-Id: If78d0104df7e18f4320c1658af75576f6a4a8c73
Reviewed-on: https://skia-review.googlesource.com/122580
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
6 files changed