[BUG][PLAYER] Slider lemmings can shimmy across the bottom boundary of the level

[Armani]

This bug is very similar to the bug that already had been fixed here: https://www.lemmingsforums.net/index.php?topic=5525.0
You can assign a shimmier to a slider lemming when he reaches the bottom boundary of the level.

[namida]

Partially fixed in commit 4d01963. Previously, an out-of-bounds check for a Shimmier (which has a more-sensitive out-of-bounds check than a lemming usually has, specifically in order to catch the edge case of "clinging to the bottom pixel of the level") was only performed if the Shimmier had moved downwards in the current frame. With a Slider being able to directly transition to a Shimmier at such a position, this check didn't trigger. I've now made it so that a Shimmier always performs this check, even if it hasn't moved downwards on the current frame.

I consider this a partial fix because I think the Shimmier assignment should be prevented outright in such a case.

[namida]

Fully fixed in commit a558308.