tiling: When displaying uncaught exceptions in the dirtyLayersWorker, use sys.excepthook()
(It's idiomatic, and allows test scripts to catch rendering and gui-thread exceptions with the same mechanism.)
ImageView2D: Provide initial values for ImageView2D.x and .y
RenderingManager: Release qvtk widget on app exit to avoid QGLContext error. See also: 3541b1dd9582b14ea51326d225e654f5bfb74f23