caopengllayer: restore vout_display_SendEventDisplaySize
VLC 4.0 doesn't need that but 3.0 still has no windowing module, leading to display size not being reported to the subtitle text renderer, and leading to blurry subtitles and OSD.
Because the size is coming from the display, it means that the first time the subtitle is displayed, it will always be blurry depending on the real display size and original media size.
Regression from 8ff56952.
Fixes #27793 (closed)