Wayland graphics support

[phlash]

Adds native Wayland graphics support, change summary:

• Minor build changes to link appropriate libraries and build new sources provided libwayland-dev and wayland-protocols packages are available (via a new CMakeModules/FindWayland.cmake helper).
• New source: src/osgViewer/GraphicsWindowWayland.cpp and supporting (generated) protocols (src/osgViewer/xdg*) to communicate with Wayland compositors.
• Minor fixes to src/osg/GraphicsContext.cpp to automatically select a working graphics backend at startup, while allowing manual selection through an environment variable OSG_WINDOWING_SYSTEM

This applies against current master and works with both osgviewer and as a patch to FlightGear (my chosen test app!) It has been independently tested by @shiptux on sway, kwin and weston compositors.