* Platform/MacOS/QuartzDisplayDeviceDriver.cs: Updated to use the new
DisplayDevice constructors.
* Platform/X11/X11XrandrDisplayDevice.cs: Query Xinerama for the exact
bounds of each display device, if available.
Use global X lock to protect from multithreaded access.
Removed OpenTK.Graphics.Glu class (reason: deprecated upstream; most functionality provided by OpenTK math; not compatible with OpenGL 3.0+.) OpenTK.Graphics.Glu can be accessed through OpenTK.Compatibility.
Added OpenGL|ES-specific ErrorHelper classes.
Moved OpenTK.Graphics.DisplayDevice and OpenTK.Graphics.DisplayResolution to the root OpenTK namespace (reason: their functionality is not specific and does not depend on OpenTK.Graphics).
Split Graphics*Exception classes into different files.
Made GraphicsErrorException public (reason: necessary for OpenTK.Compatibility).