core: add fs-screen option for fullscreen display selection

`--fs-screen` allows to decide what display to go fullscreen into. The
semantics of `--screen` changed and now it is only used to select the windowed
display when starting the application.

This is useful for people using mpv with an external TV. They will start
windowed on their laptop's screen and switch to fullscreen on the TV.

@wm4 worked on the x11 and w32 parts of the code. All is squashed in one
commit for history clarity.

1 files changed, 2 insertions, 0 deletions

diff --git a/video/out/x11_common.h b/video/out/x11_common.h
index d638d6b15f..5553c070f5 100644
--- a/video/out/x11_common.h
+++ b/video/out/x11_common.h
@@ -34,6 +34,8 @@ struct vo_x11_state {
     Window rootwin;
     int screen;
     int display_is_local;
+    int ws_width;
+    int ws_height;
 
     int screensaver_off;
     int dpms_disabled;