This adds the framework needed to check for winsys specific extensions
(such as GLX extensions) using a similar mechanism to the
cogl-feature-functions header. There is a separate
cogl-winsys-feature-functions header which will contain macros to list
the extensions and functions. cogl_create_context_winsys now calls
_cogl_feature_check for each of these functions. _cogl_feature_check
has had to be changed to accept the driver prefix as the first
parameter so that it can prepend "GLX" rather than "GL" in this case.
The Clutter X11 backend now passes all events through
_cogl_xlib_handle_event. This function can now internally be hooked
with _cogl_xlib_add_filter. These are added to a list of callbacks
which are all called in turn by _cogl_xlib_handle_event. This is
intended to be used internally in Cogl by any parts that need to see
Xlib events.
Cogl now also has an internally exposed function to set a pointer to
the Xlib display. This is stored in a global variable. The Clutter X11
backend sets this.
_cogl_xlib_handle_event and _cogl_xlib_set_display can be removed once
Cogl gains a proper window system abstraction.
This creates a separate struct to store the fields of the context that
are specific to the winsys. This is all stored in one file but ideally
this could work more like the CoglContextDriver struct and have a
different header for each winsys.
