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, 4 insertions, 0 deletions

diff --git a/video/out/opengl/common.h b/video/out/opengl/common.h
index 35d303e96c..3f5ab0ebde 100644
--- a/video/out/opengl/common.h
+++ b/video/out/opengl/common.h
@@ -244,6 +244,10 @@ struct GL {
     void (GLAPIENTRY *UniformMatrix3fv)(GLint, GLsizei, GLboolean,
                                         const GLfloat *);
 
+    GLsync (GLAPIENTRY *FenceSync)(GLenum, GLbitfield);
+    GLenum (GLAPIENTRY *ClientWaitSync)(GLsync, GLbitfield, GLuint64);
+    void (GLAPIENTRY *DeleteSync)(GLsync sync);
+
     void (GLAPIENTRY *VDPAUInitNV)(const GLvoid *, const GLvoid *);
     void (GLAPIENTRY *VDPAUFiniNV)(void);
     GLvdpauSurfaceNV (GLAPIENTRY *VDPAURegisterOutputSurfaceNV)