Tile culling logic is not optimal

[kring]

I noticed while working on #981 that our tile culling logic isn't as good as it can be.

In order for a tile to be visible, it must pass both the frustum test and the fog test. However, nothing requires that it pass both tests in the same view. So if a scene has two cameras, and in one a given tile is very far away, so it should be fog culled, and in the other the tile is nearby but behind the camera so it should be frustum culled, then this tile will be considered visible, when really it's not.