Add particle to splatted terrain

Currently you can specify which particle are emitted when you driver on the grass texture, grass particles might be emitted.

The problem is with splatting this isn't supported. Current guess is that we can use the splatting fallback to know which texture is under the kart.

Migrated-From: https://sourceforge.net/apps/trac/supertuxkart/t/ticket/1145
