osc.lua: add option to make rendering smoother

OSC rendering used to be smooth (up to OSD rendering fps) before
48f906249e, but after that commit the
frame duration is hardcoded to 30 ms. This is too high and results in
choppy OSC rendering, which is very noticeable with the progress bar
while moving mouse over it or playing a short 60 fps video.

This makes the duration an option so that it can be decreased to make
OSC rendering smoother.

DOCS/man/osc.rst

@@ -472,6 +472,12 @@ Configurable Options
 
     Sets the colors of the elements that are being pressed or held down.
 
+``tick_delay``
+    Default: 1/60
+
+    Sets the minimum interval between OSC redraws in seconds. This can be
+    decreased on fast systems to make OSC rendering smoother.
+
 
 Script Commands
 ~~~~~~~~~~~~~~~