[X11] Added dllmap for Mac OS X (XQuartz)
Merge branch 'xquartz' of https://github.com/thefiddler/opentk into xquartz
[X11] Implement support for XQuartz (X11 on Mac)
[X11] Check before monitoring /dev/input

This avoids a crash on non-Linux systems that do not support /dev/input.
[X11] Throw exception if GLX is not supported

You cannot create an X11/OpenGL context without the GLX extension.
OpenTK will now throw a `NotSupportedException` when this condition is
encountered.

In some cases, it may be possible to enable the EGL backend by passing
`GraphicsContextFlags.Embedded` to the `GraphicsContext` constructor.
[X11] Added glXQueryVersion DllImport.
[X11] Construct X11 Factory first if enabled

This allows us to use X11 on Windows and Mac, if explicitly requested
by the user via `ToolkitOptions.Backend`.
Merge pull request #135 from thefiddler/xquartz

XQuartz support (X11 on Mac)