fix anti-aliasing seams

Author: slembcke

demo/ChipmunkDebugDraw.c

@@ -126,7 +126,7 @@ ChipmunkDebugDrawInit(void)
 
 			void main(){
 				float len = length(FRAG.uv);
-				float fw = length(fwidth(FRAG.uv));
+				float fw = length(vec2(dFdx(len), dFdy(len)));
 				float mask = smoothstep(-1.0, fw - 1.0, -len);
 
 				float outline = 1.0 - fw;