This handles all border corners styles except those with
dashed and/or dotted styles. Those require changes to the
border clip mask shader, which will be done in a follow up.

The basic idea is to draw the corner twice, once for each style
and mask out the other side of the corner. This results in more
pixels being drawn than necessary, and can sometimes result in the
AA between the corner segments being slightly incorrect. Those
issues can be fixed at a later time.