vo_opengl: if glfinish is used, also call it after swappping

It seems that at least on nvidia systems with composting disabled, we
can get it to block deterministically on the actual vsync event, which
should improve framedropping.

DOCS/man/vo.rst

@@ -439,7 +439,9 @@ Available video output drivers are:
         Borders will be distorted due to filtering.
 
     ``glfinish``
-        Call ``glFinish()`` before swapping buffers
+        Call ``glFinish()`` before and after swapping buffers (default: disabled).
+        Slower, but might help getting better results when doing framedropping.
+        The details depend entirely on the OpenGL driver.
 
     ``sw``
         Continue even if a software renderer is detected.

video/out/vo_opengl.c

@@ -105,6 +105,9 @@ static void flip_page(struct vo *vo)
     if (p->frames_rendered > 5)
         gl_video_set_debug(p->renderer, false);
 
+    if (p->use_glFinish)
+        p->gl->Finish();
+
     mpgl_unlock(p->glctx);
 }