options: reduce swapchain-depth to 2 by default

It's enough to not wait for the flip and start with next frame and in
the same time minimized latency which helps display-sync modes, that are
not aware about swapchain depth.

DOCS/man/options.rst

@@ -1998,7 +1998,7 @@ Video
     missed vsyncs, but increases visible latency. This option only mandates an
     upper limit, the implementation can use a lower latency than requested
     internally. A setting of 1 means that the VO will wait for every frame to
-    become visible before starting to render the next frame. (Default: 3)
+    become visible before starting to render the next frame. (Default: 2)
 
 Audio
 -----

options/options.c

@@ -287,7 +287,7 @@ const struct m_sub_options vo_sub_opts = {
         .mmcss_profile = "Playback",
         .ontop_level = -1,
         .timing_offset = 0.050,
-        .swapchain_depth = 3,
+        .swapchain_depth = 2,
         .focus_on = 1,
     },
 };