GPv3: Improve redrawing for drawing operator

The new drawing operator uses the `paint_stroke` API which didn't
know about grease pencil yet. This simple change causes the redraw
to happen on every(?) event, which did not happen before.

source/blender/editors/sculpt_paint/paint_stroke.cc

@@ -1051,6 +1051,11 @@ bool paint_space_stroke_enabled(Brush *br, ePaintMode mode)
     return false;
   }
 
+  if (mode == PAINT_MODE_GPENCIL) {
+    /* No spacing needed for now. */
+    return false;
+  }
+
   return paint_supports_dynamic_size(br, mode);
 }