Test tile type from state transition

As I update an old post to explain that yes, cando() finally managed air/water transition I note that the whole blog is still missing a key explanation of how this was made possible. It happens in state machine transition expression (in the predicates, actually) and says "tell me the properties of the world n pixels above my hotspot".

$FALL->$INWATER on fail [2 H WATER ?]
                        (v1 v5 + 2 / :1 0 :5);
$INWATER->$RBOUNCE on event0 [D_FOOT 8 H AIR ? &]
                        (800 ~ :1);

You see it there. H is the new OP_HOTTILE for the GobExpressions. it picks the value on top of the stack, the constant 2 or 8 here. We can then test bits against the constants WATER or AIR with the ? operator.

Without that, the trick about a slice of tiles that are both water and air is useless, because you couldn't make the difference between falling into water and falling on the ground anyway.

(and yeah, I still have to add those get-worldwide-flag and set-worldwide-flag opcodes as well as the roll-the-dice opcode)