GPUParticles: Kill snow flake when it hit the ground according to the heightmap.

(Actually we move it outside of the player view)

git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@14977 178a84e3-b1eb-0310-8ba1-8eac791a3b58

data/shaders/particle.vert

@@ -34,7 +34,7 @@ void main(void)
 
 	if (hasHeightMap) {
 		float h = position.y - texelFetch(heightmap, i * 256 + j).r;
-		newposition.y = (h > 0.)? position.y : position.y - h;
+		newposition.y = (h > 0.)? position.y : 1000.;
 	}
 
 	if (flips)