display: provide a read-only version of the picture placement

Most display modules use the exact computation done in the core to place the video after window size, crop, aspect ratio, zoom changes. This value is already kept in the core to spot changes. We can share it so display modules don't have to compute it again.

It also guarantees that the video placement matches exactly the one from the core which is also used to place subpictures.

Now when VOUT_DISPLAY_CHANGE_SOURCE_PLACE is received, that means the values in vd->place have changed. Just like VOUT_DISPLAY_CHANGE_SOURCE_ASPECT and VOUT_DISPLAY_CHANGE_SOURCE_CROP mean values changed in vd->source.