///get_ground_tile(x, y);
// Returns the tile from one of the ground layers, or -1 if none found.

var xx = argument0,
    yy = argument1;

var tile = tile_layer_find(-100, xx, yy);
if(tile == -1) tile = tile_layer_find(-90, xx, yy);
if(tile == -1) tile = tile_layer_find(-99, xx, yy);
if(tile == -1) tile = tile_layer_find(-101, xx, yy);
if(tile == -1) tile = tile_layer_find(-105, xx, yy);
if(tile == -1) tile = tile_layer_find(-110, xx, yy);
if(tile == -1) tile = tile_layer_find(-120, xx, yy);

return tile;