Better support for non double buffering windows

[szabolcsdombi]

Double buffering is not necessary with zengl.
Rendering is done entirely offscreen and the final image may be blitted to the screen.
This technique does not require double buffering to hide in-progress rendering artifacts.
When double buffering is off, it seems the rendering queue is not flushed automatically at the end of a frame. (the end of the frame is not clearly defined in this case)
glFlush must be called

TODO: implement Context.flush()
TODO: maybe Image.blit(..., flush=True)
TODO: keep only one of the above

[szabolcsdombi]

flush is definitely a must have #2

[szabolcsdombi]

a much more simple solution is to flush after blitting to the screen.
the user should not be aware of a required explicit flush.