This triggers when the player steps on top of a sprite.
Sprites that use this should have "noCollide" set to true.
The SPRITE_GRID is initialized by placeSprite()

The SPRITE_GRID is a 2-dimensional array of references to sprite movieClips.
When the player steps ON TOP of a tile, the player will attempt to call the gridCollide() of whatever sprite is stored there.

Sprite positions are NOT updated when they move,
they are updated when placeSprite() is called,
so stepping on their starting tile will likely trigger their gridCollide() script if it exists.
