Temp context not necessary for GLX

GLX entry points are not bound to a specific context. This means that, unlike WGL, GLX does not require a temporary context in order to load its entry points!
Fix support for 3.2+ contexts (issue 3111)

glXGetProcAddress may return a non-null value even if a function is not
supported by the server. We need to check the extension string using
any GLX extensions. Fixes issue http://www.opentk.com/node/3111 "GLX
extension support is not checked correctly".

Furthermore, mode selection is now performed explicitly by the
X11GLContext constructor.