osc: define and add support for "thumbnailer" api

this adds a "standard" api for ui scripts and thumbnailers to
communicate with each other, based on the simple thumbfast api
[1].

the api works as follows:

* If a thumbnailer script is active, it will set
  `user-data/thumbnailer` property with the width and height of
  the thumbnail. UI scripts can observe this property to know
  the thumbnail w/h along with whether thumbnailer is active or
  not.
* To issue a thumbnail draw command, the UI script will set the property
  `user-data/osc/thumbnailer` with `hover_sec`, `x` and `y`
  field set.
* To clear the thumbnail, the UI script will set the previously
  mentioned property to `nil`.

a more ideal api would make it so that the thumbnailer script
only generates the thumbnail and doesn't need to draw at all.
but this is a decent enough api that allows arbitrary
thumbnailers and ui scripts to communicate between each other
and work together.

this change has been tested with work with thumbfast (using the
"thumbfast-glue" script below). and for demonstration that this
api can be useful outside of osc, it has also been tested to
work on mfpbar's thumbnailer branch [3].

the code to determine thumbnail x,y is based on the osc fork
inside of thumbfast [2].

[1]: https://github.com/po5/thumbfast?tab=readme-ov-file#for-ui-developers-how-to-add-thumbfast-support-to-your-script
[2]: https://github.com/po5/thumbfast/tree/vanilla-osc
[3]: https://codeberg.org/NRK/mpv-toolbox/src/branch/thumbnailer/mfpbar

Author: N-R-K

player/lua/osc.lua

@@ -191,6 +191,7 @@ local layouts = {}
 local is_december = os.date("*t").month == 12
 local UNICODE_MINUS = string.char(0xe2, 0x88, 0x92)  -- UTF-8 for U+2212 MINUS SIGN
 local last_custom_button = 0
+local thumbnailer = { width = 0, height = 0 }
 
 local function osc_color_convert(color)
     return color:sub(6,7) .. color:sub(4,5) ..  color:sub(2,3)
@@ -1063,6 +1064,28 @@ local function render_elements(master_ass)
                     ass_append_alpha(elem_ass, slider_lo.alpha, 0)
                     elem_ass:append(tooltiplabel)
 
+                    -- thumbnail
+                    local osd_w = mp.get_property_number("osd-width")
+                    if thumbnailer.width > 0 and thumbnailer.height > 0 and osd_w then
+                        local r_w, r_h = get_virt_scale_factor()
+
+                        local tooltip_font_size = (user_opts.layout == "box" or user_opts.layout == "slimbox") and 2 or 12
+                        local thumb_ty = user_opts.layout ~= "topbar" and element.hitbox.y1 - 8 or element.hitbox.y2 + tooltip_font_size + 8
+                        local thumb_tx = tx
+                        local thumb_pad = 2
+                        local thumb_margin_x = 20 / r_w
+                        local thumb_margin_y = (4 + user_opts.tooltipborder) / r_h + thumb_pad
+                        local thumb_x = math.min(osd_w - thumbnailer.width - thumb_margin_x, math.max(thumb_margin_x, thumb_tx / r_w - thumbnailer.width / 2))
+                        local thumb_y = user_opts.layout ~= "topbar" and thumb_ty / r_h - thumbnailer.height - tooltip_font_size / r_h - thumb_margin_y or thumb_ty / r_h + thumb_margin_y
+
+                        mp.set_property_native("user-data/osc/thumbnailer", {
+                            hover_sec = mp.get_property_number("duration", 0) * (sliderpos / 100),
+                            x = math.floor(thumb_x + 0.5), y = math.floor(thumb_y + 0.5)
+                        })
+                    end
+
+                else
+                    mp.set_property_native("user-data/osc/thumbnailer", nil)
                 end
             end
 
@@ -2734,6 +2757,13 @@ mp.observe_property("chapter-list", "native", function(_, list)
     update_duration_watch()
     request_init()
 end)
+mp.observe_property('user-data/thumbnailer', 'native', function(kind, data)
+    if data then
+        thumbnailer = data
+    else
+        thumbnailer = { width = 0, height = 0 }
+    end
+end)
 
 -- These are for backwards compatibility only.
 mp.register_script_message("osc-message", function(message, dur)