This method checks if the model is on screen by projecting points at the model
extremes to screen and seeing if they are in the viewport bounds.
When doing this, it offsets the model height for the top of the viewport check,
but does not offset the bottom of the model for the bottom viewport check. This
offsets the bottom height to the model y position before doing this check,
which improves the test on models which are beneath the tile they are placed on.
This is most noticible in the Karamja dungeon and Hydra dungeon which has
objects for the lava instead of tiles, and they are placed at y=0 with some or
all of the model being below that.
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