win32: allow window frame to be drawn in dark mode colors based on sy…

…stem setting

This is now documented way to allow dark mode window frame appearance:

https://learn.microsoft.com/windows/apps/desktop/modernize/apply-windows-themes
https://learn.microsoft.com/windows/win32/api/dwmapi/ne-dwmapi-dwmwindowattribute

I'm not interested in various undocumented APIs to make context menu
dark and so on.

Documentation says this DWM attribute is supported starting with Windows
11 Build 22000. It seems to work well on Windows 10... while in fact
doesn't work on Windows 11, at least on my system. Feel free to report it
to Microsoft and ping me when they release proper API for that. Until
then this small improvement is a step the right direction.

Fixes: mpv-player#6901

video/out/w32_common.c

@@ -1545,6 +1545,9 @@ static void *gui_thread(void *ptr)
         goto done;
     }
 
+    DwmSetWindowAttribute(w32->window, 20 /* DWMWA_USE_IMMERSIVE_DARK_MODE */,
+                          &(BOOL){TRUE}, sizeof(BOOL));
+
     if (SUCCEEDED(OleInitialize(NULL))) {
         ole_ok = true;