twn_draw: texture_region option for draw_billboard()

src/rendering/twn_draw_c.h

@@ -121,6 +121,8 @@ typedef struct SpaceBillboard {
     Vec3 position;
     Vec2 size;
     Color color;
+    m_option_list(
+        Rect, texture_region )
     // TextureKey texture; /* is assumed from other places */
     bool cylindrical;
 } SpaceBillboard;