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, 10 insertions, 0 deletions

diff --git a/video/out/opengl/common.c b/video/out/opengl/common.c
index f045184373..fae969ee2a 100644
--- a/video/out/opengl/common.c
+++ b/video/out/opengl/common.c
@@ -230,6 +230,16 @@ static const struct gl_functions gl_functions[] = {
         .extension = "GL_ARB_texture_rg",
         .provides = MPGL_CAP_TEX_RG,
     },
+    {
+        .ver_core = 320,
+        .extension = "GL_ARB_sync",
+        .functions = (const struct gl_function[]) {
+            DEF_FN(FenceSync),
+            DEF_FN(ClientWaitSync),
+            DEF_FN(DeleteSync),
+            {0}
+        },
+    },
     // Swap control, always an OS specific extension
     // The OSX code loads this manually.
     {