lua: fix passing non-integers to mp.set_osd_ass()

libass uses integers for PlayResX/Y, so the osd-overlay command also
does. Lua (pre-5.3) does not have an integer type, but the command
interface makes a difference anyway. If you pass a Lua number with a
fractional part to an integer parameter (using mp.command_native()), it
will result in an error and complain about incompatible types.

I think that's fine, but since this behavior extends to
mp.set_osd_ass(), this is a compatibility problem.

Fix this by explicitly coercing the resolution parameters to integer
numbers.

player/lua/defaults.lua

@@ -612,6 +612,8 @@ function overlay_mt.update(ov)
         cmd[k] = v
     end
     cmd.name = "osd-overlay"
+    cmd.res_x = math.floor(cmd.res_x)
+    cmd.res_y = math.floor(cmd.res_y)
     mp.command_native(cmd)
 end