shiny: vendor github.com/BurntSushi/xgb/...

As the README.vendor file says:

----

This is a fork-and-subset of github.com/BurntSushi/xgb at git revision
3ac861bb (2015-04-26).

The subset consists of these directories:
github.com/BurntSushi/xgb
github.com/BurntSushi/xgb/render
github.com/BurntSushi/xgb/shm
github.com/BurntSushi/xgb/xproto

Other directories in the original github.com/BurntSushi/xgb repository
are not used by golang.org/x/exp/shiny, and were not copied to the
vendor tree here:
github.com/BurntSushi/xgb/bigreq
github.com/BurntSushi/xgb/composite
etc
github.com/BurntSushi/xgb/xvmc

----

Change-Id: I7702bd31443239269b7218c368e27802fbfed78a
Reviewed-on: https://go-review.googlesource.com/12814
Reviewed-by: Andrew Gerrand <adg@golang.org>
16 files changed