reduce one use of SystemWinData

Change-Id: I777a8e324864ab9eab2a3df3e0645910dd1478a4
split method in part related to windows and other stuff

Change-Id: Ie3851bfd558ffeabd374afdc2a4d4833e3866a6e
add a OpenGLContext::init for unix backend

That one does not need the indirection through a Window instance to get
to the X resources that are necessary for a GLX context.

Change-Id: I3195a5f2b447172434881bd9b0b230c8992c1c87