The tilsets are in two parts:
upper half is hz-flip=0
lower half is hz-flip=1
for both graphics and collisions.

If selected tile is on the first half we copy its number,
in the other case because of hz-flip we have to take the
symmetic number and record that it has to be flipped.
For collisions we don't take that into account.

8 collisions types:
 * empty
 * full
 * bottom half
 * top half
 * bottom slope
 * top slope
 * side
 * special
These can be hz-flipped using the hz-flip bit.


 


