[Wayland] No borders on player's window

[Jiehong]

mpv version and platform

Mpv at ddaab33, on Archlinux.

Reproduction steps

1. Build mpv;
2. log on to Gnome 3.20 or 3.22 on wayland;
3. launch a video file smaller than the screen size with mpv.

Expected behaviour

Player opens, and video starts playing in its own window. The windows opens in the centre of the screen.

Actual behaviour

Player opens, and video starts, but the player's window has no borders on any side. It also appears on the top left corner by default, never in the centre.

Log file

See attachment.
output.txt

Sample files

Happens with any file or stream (like youtube video).

[ghost]

Duplicate of #139.

[ghost]

Duplicate of #3646