Add sdfy symmetric corner

[elcritch]

This PR changes it so each corner is generated independently and symmetrically. Then rotation is used to flip the image correctly.

While this works very efficiently it's currently resulting in very annoying visual artifacts on the resulting shadows. This is due to the rotation not being perfect, largely based on factors of PI.

I've yet to figure out how to work around this. I thought I had, by taking the floor of the centering offset and then none after that when reversing the rotation centering. The images generally seem correctly place, but the middle images are either too long or too short by 1 pixel. Sometimes they'll flip from being perfect to being off by one pixel, indicating it is a rounding issue. I suspect it's due to translating to the image center before rotating. I'd like to understand what coordinate system and rotations they're using. Maybe need to setup the transform matrix by hand? Hmmm, that could work.

I'm also still unsure as to whether the coordinates are in Top-Left (Quadrant 4) or something else.