av-sync finer tuning

[totaam]

Follow up from #835.

Things that we want to quantify because they may need to be taken into account when calculating the video sync delay:

• operating system / driver output buffering: how long it takes from the moment we send bytes to the soundcard until they actually come out of the speakers
• gstreamer internal buffering its sink element, different on each OS
• application specific internal processing delay (which we should probably ignore - the application should submit sound and video in sync to the server OS)
• picture decoding time: this is small, but should have been substracted from the total already
• opengl double-buffering vs single-buffering and framerate (we do have a value for vsync we export) - also needs to be substracted from the total
• any other display specific delay: 4k vs 1080p should not make any difference, at least I can't think of how it would - maybe related to sync paint updates to the video #981

From what I've heard, the audio is a bit late, especially on non-Linux platforms.
If that's the case, we can quite easily add per-OS tweaks.

[totaam]

Edit: ignore comment, reverted in r12870.

See also #1261 for multiple b-frames support, r12868 takes delayed frames into account when calculating the delay.

[totaam]

There may also be differences between codecs, see #1075#comment:10 : Mp3, on the other hand, seemed to be about 6-8 frames audio late.

[totaam]

As of r17223, users can tune the av-sync from the system tray.

[totaam]

See also #2517.

[totaam]

r17223 should be enough for now.