This moves us to always rendering to an off-screen texture, and then
(at idle) blitting this to the screen & swapping buffers. Ideally we
should never see any rendering, or flicker again with this approach.

Several fixes are included:
   + avoid multiple OpenGL contexts being created for the same window,
     created excessive flicker problems.
   + de-virtualize UseContext - which context we use is less critical.
   + kill 'mbOffscreen' distinction - all VCL rendering is offscreen.
   + implement 'doFlush' and high priority idle flushing.
   + bind stencil buffer for clipping vs. textures - fixing complex
     clopping when rendering to virtual-devices, and off-screen.
   + document environment. variables.
   + use white as default background glClear color, but red or
     random color for DBGUTIL.

Change-Id: I6be08595b6c8deb7e6db0dbd81308b2c97d2b4ff