Issue 247: Always render edges of selected brushes.
Add command to center the camera on the current selection.
Fix a bug in BBox intersection method.
Remove empty brush entities after reparenting brushes.
Prevent brush creation when faces are selected.
Use a drag plane that is oriented towards the camera when moving objects
vertically with the mouse.

Use the camera's up vector as the look vector when computing a billboard
matrix with fixed up vector if the user is looking straight up or down.