gdk: Add GdkDisplay::init_gl vfunc

The vfunc is called to initialize GL and it returns a "base" context
that GDK then uses as the context all others are shared with. So the GL
context share tree now looks like:

+ context from init_gl
  - context1
  - context2
  ...

So this is a flat tree now, the complexity is gone.

The only caveat is that backends now need to create a GL context when
initializing GL so some refactoring was needed.

Two new functions have been added:

* gdk_display_prepare_gl()
  This is public API and can be used to ensure that GL has been
  initialized or if not, retrieve an error to display (or debug-print).
* gdk_display_get_gl_context()
  This is a private function to retrieve the base context from
  init_gl(). It replaces gdk_surface_get_shared_data_context().

1 files changed, 3 insertions, 0 deletions

diff --git a/gdk/gdkdisplayprivate.h b/gdk/gdkdisplayprivate.h
index 4aa99c30dd..a2913b50e9 100644
--- a/gdk/gdkdisplayprivate.h
+++ b/gdk/gdkdisplayprivate.h
@@ -140,6 +140,8 @@ struct _GdkDisplayClass
 
   GdkKeymap *                (*get_keymap)         (GdkDisplay    *display);
 
+  GdkGLContext *         (*init_gl)                    (GdkDisplay        *display,
+                                                        GError           **error);
   gboolean               (*make_gl_context_current)    (GdkDisplay        *display,
                                                         GdkGLContext      *context);
 
@@ -206,6 +208,7 @@ GdkSurface *        gdk_display_create_surface        (GdkDisplay       *display
                                                        int               width,
                                                        int               height);
 
+GdkGLContext *      gdk_display_get_gl_context        (GdkDisplay       *display);
 gboolean            gdk_display_make_gl_context_current  (GdkDisplay        *display,
                                                           GdkGLContext      *context);