osc.lua: add auto value to vidscale option

This adds auto to vidscale script option, which lets the scale be
inherited from OSD --osd-scale-by-window option.

DOCS/interface-changes/osdscale.txt

@@ -0,0 +1 @@
+change `vidscale` script option type to string for osc.lua

DOCS/man/osc.rst

@@ -250,8 +250,10 @@ Configurable Options
 ``vidscale``
     Default: yes
 
-    Scale the OSC with the video
-    ``no`` tries to keep the OSC size constant as much as the window size allows
+    Scale the OSC with the video.
+    ``no`` tries to keep the OSC size constant as much as the window size allows.
+    ``auto`` scales the OSC with the OSD, which is scaled with the window or kept at a
+    constant size, depending on the ``--osd-scale-by-window`` option.
 
 ``valign``
     Default: 0.8

player/lua/osc.lua

@@ -14,7 +14,7 @@ local user_opts = {
     idlescreen = true,          -- show mpv logo on idle
     scalewindowed = 1,          -- scaling of the controller when windowed
     scalefullscreen = 1,        -- scaling of the controller when fullscreen
-    vidscale = true,            -- scale the controller with the video?
+    vidscale = "yes",           -- scale the controller with the video?
     valign = 0.8,               -- vertical alignment, -1 (top) to 1 (bottom)
     halign = 0,                 -- horizontal alignment, -1 (left) to 1 (right)
     barmargin = 0,              -- vertical margin of top/bottombar
@@ -1785,7 +1785,14 @@ function osc_init()
         scale = user_opts.scalewindowed
     end
 
-    if user_opts.vidscale then
+    local scale_with_video
+    if user_opts.vidscale == "auto" then
+        scale_with_video = mp.get_property_native("osd-scale-by-window")
+    else
+        scale_with_video = user_opts.vidscale == "yes"
+    end
+
+    if scale_with_video then
         osc_param.unscaled_y = baseResY
     else
         osc_param.unscaled_y = display_h