Fix undefined behaviour introduced in rP22473

rP22473 introduces undefined behaviour: for vertices that are part of non-AA squares, the value of "quadOutward" is undefined. This fixes that by again setting it to QUADRANT_ALL by default, as it was pre rP22473. This led to OOS.

Reviewed By: elexis

Fixes #5508

Differential Revision: https://code.wildfiregames.com/D2082