TerrainOverlay should not use a global rendering list

[vts]

The TerrainOverlay class is used to render mostly debugging overlays for e.g. the pathfinder and the territory manager. Currently, it uses a global list of overlays to be rendered, with some provisions for controlling the drawing order by assigning priorities.

The fact that a global list is used may potentially cause problems in Atlas, which spawns multiple simulation instances each with its own terrain. For instance, the ActorViewer creates its own (tiny) terrain to preview the selected actor on. As a result, any overlays for the main terrain will also be rendered on top of the actor viewer's terrain, and vice-versa.

Because TerrainOverlays are primarily used for debugging purposes, and people usually don't have debugging overlays enabled in Atlas, this problem isn't likely to affect many people, hence the low priority. Would still be nice to fix this though.