shiny/driver/gldriver: delay showWindow call
shiny/driver/gldriver: delay showWindow call

This change snuck into cl/14743 and breaks drawing on OS X.

Change-Id: I6ead61b68a1a98959e646c29a2a7f61768001b18
Reviewed-on: https://go-review.googlesource.com/14791
Reviewed-by: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>

golang/exp,AkihiroSuda/golang-exp-aspectgo,golang/exp,JamesDunne/exp,JamesDunne/exp,golang/exp,AkihiroSuda/golang-exp-aspectgo,AkihiroSuda/golang-exp-aspectgo