Expose EGL context/display and GLX context

To help facilitate integration with third party frameworks this exposes
the EGL context and display to applications as well as the GLX context.
(Note that the GLX display is already available via
cogl_xlib_renderer_get_display())

This adds two new top-level headers <cogl/cogl-egl.h> and
<cogl/cogl-glx.h> that need to be included explicitly to access these
apis.

Anyone using these apis will be responsible for checking that Cogl
is indeed using EGL or GLX by calling cogl_renderer_get_winsys_id()

This will enable GStreamer, for example, to be able to create a GL
context that shares resources with Cogl's context.

https://bugzilla.gnome.org/show_bug.cgi?id=724992
Expose WGL context to applications

This exposes the WGL HGLRC context handle to applications.

This will enable GStreamer, for example, to be able to create a GL
context that shares resources with Cogl's context.

https://bugzilla.gnome.org/show_bug.cgi?id=724992