Commits on Dec 23, 2015

1. Fix a couple of correctness issues around border-radius. …

   1. Masks for large border radii were not being tessellated properly.
   This commit simply switches off tessellation for them. Eventually we may
   want to go back and fix this properly.
   
   2. Border corners were being rendered correctly when 0 < border radius <
   border width. We needed to do a bit more tessellation than we were. This
   commit fixes that and adds a diagram to `add_border_corner` to explain
   what's going on.
   
   These commits fix the `border_radius_clip_a.html` and
   `border_radius_zero_sizes_a.html` reftests.

   

   pcwalton committed Dec 23, 2015
   8623bdd