vo_opengl: add vsync-fences option

Yet another relatively useless option that tries to make OpenGL's sync
behavior somewhat sane. The results are not too encouraging. With a
value of 1, vsync jitter is gone on nVidia, but there are frame drops
(less than with glfinish). With 2, I get the usual vsync jitter _and_
frame drops.

There's still some hope that it might prevent too deep queuing with some
GPUs, I guess.

The timeout for the wait call is 1 second. The value is pretty
arbitrary; it should just not be too high to freeze the process (if
the GPU is un-nice), and not too low to trigger the timeout in normal
cases, even if the GPU load is very high. So I guess 1 second is ok
as a timeout.

The idea to use fences this way to control the queue depth was stolen
from RetroArch:

https://github.com/libretro/RetroArch/blob/df01279cf318e7ec90ace039d60515296e3de908/gfx/drivers/gl.c#L1856

1 files changed, 9 insertions, 0 deletions

diff --git a/DOCS/man/vo.rst b/DOCS/man/vo.rst
index 7b6358be6e..2cebd153ff 100644
--- a/DOCS/man/vo.rst
+++ b/DOCS/man/vo.rst
@@ -667,6 +667,15 @@ Available video output drivers are:
 
         X11/GLX only.
 
+    ``vsync-fences=<N>``
+        Synchronize the CPU to the Nth past frame using the ``GL_ARB_sync``
+        extension. A value of 0 disables this behavior (default). A value of
+        1 means it will synchronize to the current frame after rendering it.
+        Like ``glfinish`` and ``waitvsync``, this can lower or ruin performance.
+        Its advantage is that it can span multiple frames, and effectively limit
+        the number of frames the GPU queues ahead (which also has an influence
+        on vsync).
+
     ``dwmflush=<no|windowed|yes>``
         Calls ``DwmFlush`` after swapping buffers on Windows (default: no).
         It also sets ``SwapInterval(0)`` to ignore the OpenGL timing. Values