compositor: Force "GL_MaxFramesAllowed" to 1

Bug: 15453

NVIDIA proprietary/closed source driver queues up to 2 frames by default
before blocking in glXSwapBuffers(), whereas our compositor expects
`glXSwapBuffers()` to block until the next vblank.

To avoid that, our compositor is issuing a `glXWaitGL()` immediately
after the call to `glXSwapBuffers()` but that translates as a busy wait,
hence dramatically increasing CPU usage of xfwm4 with the NVIDIA
proprietary/closed source driver.

Instruct the NVIDIA proprietary/closed source driver to allow only 1
frame using the environment variable “__GL_MaxFramesAllowed” so that it
matches our expectations.

Taken from similar patch posted by NVIDIA developer for kwin:
https://phabricator.kde.org/D19867



Signed-off-by: Olivier Fourdan <fourdan@xfce.org>