win32: support the property 'focused'

And also change the existing WM_KILLFOCUS handler to return 0 instead
of 'break' (which later calls DefWindowProcW), as MSDN says we should
do for WM_{KILL,SET}FOCUS.

It seems that the 'focused' property is now supported by all main VOs:
x11, macOS, wayland, Windows.

TCT/sixel/caca probably don't support it, and unknown with SDL.

Fixes #8868

DOCS/man/input.rst

@@ -2502,8 +2502,7 @@ Property list
     ways. The property is unavailable if no video is active.
 
 ``focused``
-    Whether the window has focus. Currently works only on X11, Wayland and
-    macOS.
+    Whether the window has focus. Might not be supported by all VOs.
 
 ``display-names``
     Names of the displays that the mpv window covers. On X11, these

video/out/w32_common.c

@@ -97,6 +97,8 @@ struct vo_w32_state {
     // Has the window seen a WM_DESTROY? If so, don't call DestroyWindow again.
     bool destroyed;
 
+    bool focused;
+
     // whether the window position and size were intialized
     bool window_bounds_initialized;
 
@@ -1210,7 +1212,13 @@ static LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam,
         break;
     case WM_KILLFOCUS:
         mp_input_put_key(w32->input_ctx, MP_INPUT_RELEASE_ALL);
-        break;
+        w32->focused = false;
+        signal_events(w32, VO_EVENT_FOCUS);
+        return 0;
+    case WM_SETFOCUS:
+        w32->focused = true;
+        signal_events(w32, VO_EVENT_FOCUS);
+        return 0;
     case WM_SETCURSOR:
         // The cursor should only be hidden if the mouse is in the client area
         // and if the window isn't in menu mode (HIWORD(lParam) is non-zero)
@@ -1785,6 +1793,9 @@ static int gui_thread_control(struct vo_w32_state *w32, int request, void *arg)
             return p->len ? VO_TRUE : VO_FALSE;
         }
         return VO_FALSE;
+    case VOCTRL_GET_FOCUSED:
+        *(bool *)arg = w32->focused;
+        return VO_TRUE;
     }
     return VO_NOTIMPL;
 }