x11: fix --wid=0

Another fallout resulting from the changes whether or not to wait for
mapping the window. In this case, it obviously makes no sense to wait
for mapping, because the root window is always mapped. Mapping will
never happen, and it would wait forever.

Fixes #1139.
CC: @mpv-player/stable
manpage: remove non-existing ratio-pos property

Use percent-pos instead, which is exactly the same, except with the
range 0.0-100.0.

I'm not sure how this got there; it was probably introduced and then
removed again as percent-pos got more precise.

CC: @mpv-player/stable