Allow libmpv frontends for better window-dragging implementation

For a libmpv frontend to fully support 3rd party OSCs like uosc and support window-dragging, it's necessary to know if the mouse is inside an OSC menu, in this case dragging is undesired.

I couldn't find a way to get this information, so I use a workaround, I simply do no dragging when the mouse is near a window border because OSC menus are usually found near window borders.

If the menu is in the middle, like the context menu of uosc, dragging is still performed. I didn't know uosc is very popular and doesn't fully work in mpv.net until a recent bug report, it works now after delaying the start of the drag operation for a few pixels.

So currently there are two minor defects in my frontend, dragging works only in the middle of the window, not near borders, and if there is a menu in the middle of the window dragging is still performed.

I looked at the mpv code today and what I found is that it appears to hit test here:

static bool test_mouse(struct input_ctx *ictx, int x, int y, int rej_flags)
{
    input_lock(ictx);
    bool res = false;
    for (int i = 0; i < ictx->num_active_sections; i++) {
        struct active_section *as = &ictx->active_sections[i];
        if (as->flags & rej_flags)
            continue;
        struct cmd_bind_section *s = get_bind_section(ictx, bstr0(as->name));
        if (s->mouse_area_set && test_rect(&s->mouse_area, x, y)) {
            res = true;
            break;
        }
    }
    input_unlock(ictx);
    return res;
}

Now if there was a hit test function either in client.h or as input command, it would solve my problem, or maybe there is already something I could use, or something simpler.

Start with the test_mouse function shown in the issue and inspect client.h for an existing frontend-facing hit-test entry point. Determine how an OSC menu's mouse area could be exposed to libmpv frontends, then verify that a frontend can distinguish menu interaction from window dragging, including menus near borders and in the window center.