From Roni Zanolli,Added use of Qt's new devicePixelRatio() method to assist with handling the upscalling of resolution on iPhone for "retina" displays.

From Robert Osfield, changed Roni's code to use a #define GETDEVICEPIXELRATIO to access the versioned Qt devicePixelRatio() method to avoid duplication of the Qt version checking.
From Kristofer Tingdahl, reimplement of AntiSquish node to avoid the use of an update callback.

From Robert Osfield, small ammendments to clean up header.
Merge branch 'master' of https://github.com/openscenegraph/osg into Dev