Hmm. Looked into it slightly and found a workaround at least.
Not sure if it’s a Unity, SpriteShapeRenderer bug, but it’s passing a random value for the _MainTexture_texelSize.zw. You can see it in the frame debugger, randomly changing every frame. The .xy components seem to be passed correctly though. I thought this was hardcoded into the Unity renderer, but maybe each rendering component implements it separately?
The texel size variable passes texture size information to the shader. It’s a float4 that contains (1/width, 1/height, width, height). Since .xy is passed correctly, you can just divide by .xy instead of multiplying by .zw. Open up RetroAA.cginc and fix the first line of the function there. It should be the only place you need to fix.
I made a bug report here:
https://fogbugz.unity3d.com/default.asp?1099636_s2i45t0rg5ngl6vg
and posted it to the SpriteShape forum here:
SpriteShape Preview Package page-4#post-3877081