Rework use_texture_alpha to provide three opaque/clip/blend modes

The change that turns nodeboxes and meshes opaque when possible is kept,
as is the compatibility code that warns modders to adjust their nodedefs.

builtin/game/features.lua

@@ -18,6 +18,7 @@ core.features = {
 	pathfinder_works = true,
 	object_step_has_moveresult = true,
 	direct_velocity_on_players = true,
+	use_texture_alpha_string_modes = true,
 }
 
 function core.has_feature(arg)