Update mobj floorz/ceilingz after a bustable FOF is destroyed

Fixes the first bug described in #984.

added Bug label

added small label

added 804 commits

• 1a6e48de...a68440c4 - 803 commits from branch next
• 277b64f2 - Merge branch 'next' into bustablemobjzfix

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added Movement/physics label

added 347 commits

• 277b64f2...106a98ce - 346 commits from branch next
• be6b7ca1 - Merge branch 'next' into bustablemobjzfix

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merged

mentioned in commit 050da08c