Nodes: Add Exclusion color mix mode

Expands Color Mix nodes with new Exclusion mode.

Similar to Difference but produces less contrast.

Requested by Pierre Schiller @3D_director and
@OmarSquircleArt on twitter.

Differential Revision: https://developer.blender.org/D16543

source/blender/freestyle/intern/python/BPy_Freestyle.cpp

@@ -91,6 +91,9 @@ static int ramp_blend_type(const char *type)
   if (STREQ(type, "DIFFERENCE")) {
     return MA_RAMP_DIFF;
   }
+  if (STREQ(type, "EXCLUSION")) {
+    return MA_RAMP_EXCLUSION;
+  }
   if (STREQ(type, "DARKEN")) {
     return MA_RAMP_DARK;
   }