Add NewGRF object property/flag to use land as object ground sprite

This handles variable ground densities, snow/desert, etc.

src/newgrf_extension.h

@@ -32,6 +32,7 @@ enum Action0RemapPropertyIds {
 	A0RPI_SIGNALS_ENABLE_RESTRICTED_SIGNALS,
 	A0RPI_SIGNALS_ENABLE_SIGNAL_RECOLOUR,
 	A0RPI_SIGNALS_EXTRA_ASPECTS,
+	A0RPI_OBJECT_USE_LAND_GROUND,
 };