osc.lua: seekbar hover: speed-up chapter access

This speeds up chapter name access while hovering the seekbar:

- Fetch the chapter-list property using observation rather than on
  every render while hovering the bar. Property access, especially
  with potentially lots of values (chapters), can be expensive-ish,
  and can involve locking, so the less per-render the better.

- Sort the list once (on change) to simplify the iteration logic when
  searching the chapter which matches the mouse position. It's still
  O(N) (no binary search currently), but with less work.

The cached sorted list is stored at the state. While there are other
places which access this property and could use this cache, for now we
don't touch them, because they're not performance-critical (on init
for the chapter markers, on ch_{prev,next} button click for OSD).

Caching properties using observation instead of using mp.get_property
can indeed speedup other places too, but it should be part of a system
rather than implemented per-property. Maybe some day.

player/lua/osc.lua

@@ -124,6 +124,7 @@ local state = {
     border = true,
     maximized = false,
     osd = mp.create_osd_overlay("ass-events"),
+    chapter_list = {},                      -- sorted by time
 }
 
 local window_control_box_width = 80
@@ -604,16 +605,13 @@ end
 
 -- returns nil or a chapter element from the native property chapter-list
 function get_chapter(possec)
-    local cl = mp.get_property_native("chapter-list", {})
-    local ch = nil
+    local cl = state.chapter_list  -- sorted, get latest before possec, if any
 
-    -- chapters might not be sorted by time. find nearest-before/at possec
-    for n=1, #cl do
-        if possec >= cl[n].time and (not ch or cl[n].time > ch.time) then
-            ch = cl[n]
+    for n=#cl,1,-1 do
+        if possec >= cl[n].time then
+            return cl[n]
         end
     end
-    return ch
 end
 
 function render_elements(master_ass)
@@ -2687,7 +2685,10 @@ mp.register_event("shutdown", shutdown)
 mp.register_event("start-file", request_init)
 mp.observe_property("track-list", nil, request_init)
 mp.observe_property("playlist", nil, request_init)
-mp.observe_property("chapter-list", nil, function()
+mp.observe_property("chapter-list", "native", function(_, list)
+    list = list or {}  -- safety, shouldn't return nil
+    table.sort(list, function(a, b) return a.time < b.time end)
+    state.chapter_list = list
     update_duration_watch()
     request_init()
 end)