Curves: disable stroke spacing for curve sculpt brushes

This makes the brushes more smooth, because the brush has an
effect after every mouse move, instead of only every x pixels.

For this to work well, the brushes have to look at the stroke
segments instead of at the mouse positions separately.

A more fine grained check might be added in the future.

source/blender/editors/sculpt_paint/paint_stroke.c

@@ -1000,6 +1000,10 @@ bool paint_space_stroke_enabled(Brush *br, ePaintMode mode)
     return true;
   }
 
+  if (mode == PAINT_MODE_SCULPT_CURVES) {
+    return false;
+  }
+
   return paint_supports_dynamic_size(br, mode);
 }