Fix T66233: Grease Pencil Swirl Effect not working at origin

If the control object is in the origin the radius is wrongly calculated because the result is 0.

Now, this value is clamped to avoid this situation

source/blender/draw/engines/gpencil/shaders/fx/gpencil_fx_swirl_frag.glsl

@@ -42,7 +42,11 @@ void main()
   vec2 tc = uv - center;
 
   float dist = length(tc);
-  float pxradius = (ProjectionMatrix[3][3] == 0.0) ? (radius / (loc.z * defaultpixsize)) :
+  float locpixsize = abs((loc.z * defaultpixsize));
+  if (locpixsize == 0) {
+    locpixsize = 1;
+  }
+  float pxradius = (ProjectionMatrix[3][3] == 0.0) ? (radius / locpixsize) :
                                                      (radius / defaultpixsize);
   pxradius = max(pxradius, 1);