WM: space, region type filtering for paint cursor

Avoids calling poll on mouse-move for unrelated space/region types.

source/blender/windowmanager/wm.h

@@ -44,6 +44,9 @@ typedef struct wmPaintCursor {
 
 	bool (*poll)(struct bContext *C);
 	void (*draw)(bContext *C, int, int, void *customdata);
+
+	short space_type;
+	short region_type;
 } wmPaintCursor;