x/exp/shiny/driver/x11driver: implement back buffer

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?

1.6

1. What operating system and processor architecture are you using (go env)?

$ go env 
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/eaburns"
GORACE=""
GOROOT="/home/eaburns/src/go.googlesource.com/go"
GOTOOLDIR="/home/eaburns/src/go.googlesource.com/go/pkg/tool/linux_amd64"
GO15VENDOREXPERIMENT="1"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0"
CXX="g++"
CGO_ENABLED="1"

1. What did you do?

Use shiny x11driver

1. What did you expect to see?

Double buffering.

1. What did you see instead?

No double buffering.