Add flag to action 433 to force MFE_VERTICALFLIP

This covers up an issue with !1808 (merged)'s "sticky ceiling" feature, implemented to make ERZ2's area featuring that gimmick support taller characters.

When walking out of a "sticky ceiling" area into a continuously triggering gravity flip, necessary for going from temporary to permanent reverse gravity, MFE_VERTICALFLIP was lost for 1 tic which caused the player to flip around for a split second. This branch allows the gravity flip action to force MFE_VERTICALFLIP, to sidestep that issue.

Ideally, this would be fixed at the core, but fundamentally changing how reverse gravity works is out of scope for a 2.2 patch, and this workaround is localized enough that it shouldn't affect any existing maps.

GIF recording of the 1 tic gravity flip bug: