Fix the offscreen target indicator flickering when targeting a subsystem on a distant ship

[Goober5000]

A bug report described the indicator flickering between different shapes when a subsystem defined by a POF special point was targeted on a far away ship. The cause is in HudGaugeOffscreen::calculatePosition, which could return without writing its outputs: when the eye vertex overflowed, or when the clipped point landed a pixel or two inside the clip rectangle. The caller drew anyway, so the triangle was placed at an uninitialized position with an uninitialized direction, both changing every frame.

The clip against the view frustum should leave the point exactly on a screen edge, and the edge tests allow only about one pixel of slack. Precision degrades with distance because the point is interpolated along a segment tens of kilometres long and ends up near the view apex, so at long range it can drift inside the edge and fall through.

calculatePosition now returns whether it found a position, and both its callers (the gauge and the Lua drawOffscreenIndicator function) skip drawing when it did not. The inside case falls back to the nearest screen edge, since the point is already known to be off screen, so the indicator stays steady rather than blinking.

Two related fixes found along the way:

• The current target's display list entries stored the hull centre while the projected point was the subsystem, so the offscreen triangle separation was computed for a different point than the one clipped against. They now store the subsystem position.
• g3_project_vertex used & instead of | when a vertex is at zero depth, so neither PF_OVERFLOW nor PF_PROJECTED was ever set on that path.