[Monocle] Use hardware double-buffering in software rendering

Currently when we use software rendering on embedded platforms, the software uploading pipeline is used. This pipeline sends the pixel contents of each view as a separate notification to the View on the application thread. The result in the current Lens DirectFB port is:

1. The window stack must be composed by DirectFB on every call to uploadPixels - once per window per frame.
2. There are flickering issues when the window composition happens, since it is not aligned with vsync.

Item (2) can probably be solved in the DirectFB port of Lens. Item (1) doesn't have a solution in the current implementation.

I propose having the rendering pipeline post notification of rendering completion the the Glass Application at the completion of rendering. This will enable Lens or Monocle to buffer up the window updates it receives and then to do a single composition. The push to screen then be a hardware buffer swap aligned with vsync.