Pyglet:
  - fixed timers with interval==0
GLUT:
  - new timer implementation based on glutIdleFunc instead of glutTimerFunc.
    (glutTimerFunc seems to take precedence over all other events; thus
    fast timers can block the application)
  - app tracks windows to allow quit
  - removed glutHideWindow; it was causing subsequent calls to show() to be ignored
  - GL initialization is now called by glutTimerFunc immediately after __init__.
    This seems to mimic other backends more closely.