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