Fix CA_BOUNCE height when flipped (Fixes #428)

This merge request fixes P_DoAbilityBounce(player,changemomz) always using the max of two values in all cases, where it should instead be using min while upside-down; fixes #428 (closed).
Also added some blank lines to make the function (subjectively-)nicer to look at, and moved prevmomz initialization into the if (changemomz) block.

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TL;DR (for changelog authors):
Fixed tail-bouncing height in reverse gravity

changed the description

Before merging this, !1366 (merged) should be checked out first, since that includes a fix within the refactoring.

edit: oops I didn't read the whole issue

changed title from Fix CA_BOUNCE when flipped (Fixes #428 (closed)) to Fix CA_BOUNCE height when flipped (Fixes #428 (closed))

changed the description

This merge request is about the tail-bouncing height being wrong in reverse gravity, touching nothing related to bustable FOFs (while !1366 (merged) doesn't touch bouncing height).

I've edited this merge request's title and description to mention "bounce height", not just "bounce", now, so it should be more clear from a glance about what is being fixed here.

I thought there was for some reason a bug where breaking blocks didn't work in gravflip, which I assumed because the MR's description was vague

added Bug label

merged

mentioned in commit fc1bc94d