fix BadMatch error when embedding on some windows
When embedded, st fails with BadMatch error if the embedder's window has
non-default colormap/depth/visual. This commit fixes that by creating
st's window inside root and then reparent it into embedder.
The reference window for dc.gc is also changed to match root's visuals.
A similar commit had been made for dmenu[1].
See this issue[2] on github for context.
[1]: https://git.suckless.org/dmenu/commit/
0fe460dbd469a1d5b6a7140d0e1801935e4a923b.html
[2]: https://github.com/phillbush/xfiles/issues/47