add option to BLI_scanfill_calc() - BLI_SCANFILL_CALC_HOLES, gives some speedup for BMesh ngons which never have holes and ensures predictable triangle count (totvert - 2), which is needed for pre-calculating array size.

source/blender/blenlib/BLI_scanfill.h

@@ -101,6 +101,10 @@ enum {
 	 * Assumes ordered edges, otherwise we risk an eternal loop
 	 * removing double verts. - campbell */
 	BLI_SCANFILL_CALC_REMOVE_DOUBLES   = (1 << 1),
+
+	/* note: This flag removes checks for overlapping polygons.
+	 * when this flag is set, we'll never get back more faces then (totvert - 2)*/
+	BLI_SCANFILL_CALC_HOLES            = (1 << 2)
 };
 
 int BLI_scanfill_begin(ScanFillContext *sf_ctx);