This function is similar to STATE_engine_2D_point_to_3D() only that
For the cases that we just need to know the direction in the world that our mouse (2d point)
is pointing it is enough to call STATE_engine_2D_point_to_3D_point_on_sphere()
If however we need to know the object or the polygon that is pointed by the 2D point then
we should call STATE_engine_2D_point_to_3D() instead.
Note that STATE_engine_2D_point_to_3D_point_on_sphere() is much faster then STATE_engine_2D_point_to_3D()
An example:
A player in a game holds a shotgun. when the player moves the mouse we want to set the
direction of the shotgun accordingly. To do so it is enough to call STATE_engine_2D_point_to_3D_point_on_sphere()
and call the slower function STATE_engine_2D_point_to_3D() only when the player make a shot (only then we will need to know
what object / polygon in the world got hit.
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