First phase of updating Tetragon's screen/view structure to have more
optimal integration of Stage3D.

Added new version of ScreenManager class. This version takes care of
Stage3D and creating the Context3D. It also takes care of debug
facilities creation.
Completed updating ScreenManager. View and Screen classes stay as they
are for now.
Renamed old and new screen manager classes.
Wired up Render2D with 3D context from ScreenManager.
Added Render2D and Context3D shortcuts to important Render2D classes.

Like this, it's now possible to process texture resources before
Render2D is instantiated.