D19867: [X11] Force glXSwapBuffers to block with NVIDIA driver

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REVISION SUMMARY
  The NVIDIA implementation of glXSwapBuffers will, by default, queue up
  to two frames for presentation before blocking. KWin's compositor,
  however, assumes that calls to glXSwapBuffers will always block until
  the next vblank when rendering double buffered. This assumption isn't
  valid, as glXSwapBuffers is specified as being an implicit glFlush,
  not an implicit glFinish, and so it isn't required to block. When this
  assumption is violated, KWin's frame timing logic will
  break. Specifically, there will be extraneous calls to
  setCompositeTimer with a waitTime of 0 after the non-blocking buffer
  swaps, dramatically reducing desktop responsiveness.  To remedy this,
  a call to glXWaitGL was added by Thomas Luebking after glXSwapBuffers
  in 2015 (see bug 346275, commit
  8bea96d7018d02dff9462326ca9456f48e9fe9fb <https://phabricator.kde.org/R108:8bea96d7018d02dff9462326ca9456f48e9fe9fb>).  That glXWaitGL call is
  equivalent to a glFinish call in direct rendering, so it was a good
  way to make glXSwapBuffers behave as though it implied a glFinish
  call.
  
  However, the NVIDIA driver will by default do a busy wait in glFinish,
  for reduced latency. Therefore that change dramatically increased CPU
  usage. __GL_YIELD can be set to USLEEP (case insensitive) to change
  the behavior and use usleep instead. When using the NVIDIA driver,
  KWin will disable vsync entirely if __GL_YIELD isn't set to USLEEP
  (case sensitive, a bug in KWin).
  
  However, the NVIDIA driver supports another environment variable,
  __GL_MaxFramesAllowed, which can be used to control how many frames
  may be queued by glXSwapBuffers.  If this is set to 1 the function
  will always block until retrace, in line with KWin's expectations.
  This allows the now-unnecessary call to glXWaitGL to be removed along
  with the logic to conditionally disable vsync, providing a better
  experience on NVIDIA hardware.

TEST PLAN
  Run KWin using the X11 backend with the proprietary NVIDIA driver.
  Ensure the TripleBuffer option is not set to true in Xorg's configuration file.
  Ensure vsync has not been manually disabled (either in NVIDIA's or KWin's settings).
  
  - Moving windows should not exhibit significant lag
  - CPU usage should never be excessively high
  - No screen tearing should be observed

REPOSITORY
  R108 KWin

REVISION DETAIL
  https://phabricator.kde.org/D19867

AFFECTED FILES
  plugins/platforms/x11/standalone/glxbackend.cpp

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