Add: RectPadding type.

This is similar to Rect but specifies padding to apply to a Rect.

src/core/geometry_func.cpp

@@ -48,3 +48,5 @@ Rect BoundingRect(const Rect &r1, const Rect &r2)
 
 	return r;
 }
+
+const RectPadding RectPadding::zero = {0, 0, 0, 0};