Rework the interaction between the Cogl and GDK / X11 backends.

Rework the interaction between the Cogl and GDK / X11 backends.

Previously, the Cogl backend was at times a subclass of the X11
backend, and at times a standalone one. Now it is the other way
round, with GDK and X11 backends providing the concrete classes,
layered on top of the generic Cogl backend. A new EglNative backend
was introduced for direct to framebuffer rendering. This greatly
simplifies the API design (at the expense of some casts needed)
and reduces the amount of #ifdefs, without duplicating code.

https://bugzilla.gnome.org/show_bug.cgi?id=657434