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, 4 insertions, 0 deletions

diff --git a/gdk/gdkdisplay.h b/gdk/gdkdisplay.h
index 97d8e7eee6..9fabac6b50 100644
--- a/gdk/gdkdisplay.h
+++ b/gdk/gdkdisplay.h
@@ -68,6 +68,10 @@ gboolean    gdk_display_is_rgba            (GdkDisplay  *display);
 GDK_AVAILABLE_IN_ALL
 gboolean    gdk_display_supports_input_shapes (GdkDisplay    *display);
 
+GDK_AVAILABLE_IN_4_4
+gboolean    gdk_display_prepare_gl         (GdkDisplay  *self,
+                                            GError     **error);
+
 GDK_AVAILABLE_IN_ALL
 GdkDisplay *gdk_display_get_default (void);