Rocketeer shadows are drawn over top of other units/structures without regard to height

[ghost]

The shadow is drawn at a fixed distance below the rocketeer, and this looks quite weird over a tall structure like a prism tower.

In the absence of properly implemented shadows, perhaps it can be implemented so that a shadow is not drawn over top of an object that is, horizontally, closer to the camera than the flying unit?

Revision: 41d9a20

[phrohdoh]

This ought to be solvable properly once structure height is implemented.