Add a corner segment (if valid) to the list of
border segments for this primitive.
Add an edge segment (if valid) to the list of
border segments for this primitive.
Build the set of border instances needed to draw a border
segment into the render task cache.
Create the set of border segments and render task
cache keys for a given CSS border.
Computes the maximum scale that we allow for this set of border parameters.
capping the scale will result in rendering very large corners at a lower
resolution and stretching them, so they will have the right shape, but
blurrier.