Tiles contiguous colliders mode

[quinton-ashley]

Reduce the amount of sprites that the Tiles constructor generates by combining adjacent static colliders. This would greatly improve performance. @ Tezumies had to do this manually to get his game to run at a nice frame rate on his computer.

It could be implemented as the new default Tiles constructor mode or as a user defined mode.

Alternatively, this could be implemented more broadly as a new collider type for groups group.collider = 'contiguous'