LineArt: Tolerance for faces perpendicular to view

This is due to cam->obmat precision issue,
where it affects view vector precision.

Reviewed by Sebastian Parborg (zeddb)

Differential Revision: https://developer.blender.org/D11461

source/blender/gpencil_modifiers/intern/lineart/lineart_cpu.c

@@ -2064,7 +2064,11 @@ static bool lineart_triangle_edge_image_space_occlusion(SpinLock *UNUSED(spl),
   dot_r = dot_v3v3_db(Rv, tri->gn);
   dot_f = dot_v3v3_db(Cv, tri->gn);
 
-  if (!dot_f) {
+  /* NOTE(Yiming): When we don't use `dot_f==0` here, it's theoretically possibile that _some_
+   * faces in perspective mode would get erroneously caught in this condition where they really are
+   * legit faces that would produce occlusion, but haven't encountered those yet in my test files.
+   */
+  if (fabs(dot_f) < FLT_EPSILON) {
     return false;
   }