EMI: Sprite rotation is necessary to finish the game.

I was not around for the discussion in #759, but sprites *must* be rotated occasionally.
For example, the puzzle in mot.set requires the player to read the hands of a clock (which are sprites) in order to determine where to go. Always rendering sprites head-on as in 09466c5 prevents this puzzle from working.

Other examples are more aesthetic: the ship/sky in wed.set and balloons in lua.set have water_float applied to them, which moves the sprites *and* rotates them by a few degrees.

We need to render sprites such that they are always perpendicular to the camera vector (or parallel with the clipping planes, if you prefer), while still allowing them to rotate about the camera.

The commit in this PR fixes the clock in mot.set by not skipping the rotation for overworld sprites, but it is not a structural solution.

Any ideas?