Draw lines on top of everything

[pvutov]

We have several graphical elements that need to be drawn on top of everything - sandman's pathfinding debug lines, the targeting overlay and the vision measuring overlay (T and R hotkeys).

Previously we ensured they're drawn on top of everything using a second camera with the 'OnTop' culling mask. This was disabled to improve performance (avoid rendering everything twice). We should get those elements to show up on top of everything again by making them into UI elements or using a shader or some other solution.