Inverted ProjectPositionToScreen

Need to be able to get position in world (as a vector) of a point on user screen. This should work for any type of camera.

This is not exactly inverted ProjectPositionToScreen, because real inversion would give us geometric position of all in-world positions, which could be projected at the point on screen, which is basically a half of a line. But we can derive that easily. If we take camera position and result of “Inverted ProjectPositionToScreen”, we can use it to derive direction in which user “points” with the mouse (for example).