renderborder: Make simple border case more obvious

renderborder: Make simple border case more obvious

When reaching this point, it is impossible that all border styles are
HIDDEN or NONE, but up to 3 of them can still be that style. In any
case, the "none or solid" border style is the most common on there is,
so try to make this simpler here by just appending a border node
directly instead of going through the snapshot_border path.