fix BadMatch error when embedding on some windows

fix BadMatch error when embedding on some windows

When embedded into another window, dmenu will fail with the BadMatch
error if that window have not the same colormap/depth/visual as the
root window.

That happens because dmenu inherits the colormap/depth/visual from
its parent, but draws on a pixmap created based on the root window
using a GC created for the root window (see drw.c).  A BadMatch will
occur when copying the content of the pixmap into dmenu's window.

A solution is to create dmenu's window inside root and then reparent
it if embeded.

See this mail[1] on ports@openbsd.org mailing list for context.

[1]: https://marc.info/?l=openbsd-ports&m=168072150814664&w=2