[gtk+] gl: Add a realize() fallback

[Emmanuele Bassi <ebassi src gnome org>]

commit fe684078f9f76d71c3f771a38d287ebc255eaa8e
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Mon May 23 08:31:47 2016 +0100

    gl: Add a realize() fallback
    
    Calling gdk_gl_context_realize() should always result in a valid result,
    so we need to provide a default implementation, to avoid a call to a
    NULL function pointer.

 gdk/gdkglcontext.c |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)
---
diff --git a/gdk/gdkglcontext.c b/gdk/gdkglcontext.c
index 44ef88b..40addbc 100644
--- a/gdk/gdkglcontext.c
+++ b/gdk/gdkglcontext.c
@@ -286,11 +286,23 @@ gdk_gl_context_upload_texture (GdkGLContext    *context,
     }
 }
 
+static gboolean
+gdk_gl_context_real_realize (GdkGLContext  *self,
+                             GError       **error)
+{
+  g_set_error_literal (error, GDK_GL_ERROR, GDK_GL_ERROR_NOT_AVAILABLE,
+                       "The current backend does not support OpenGL");
+
+  return FALSE;
+}
+
 static void
 gdk_gl_context_class_init (GdkGLContextClass *klass)
 {
   GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
 
+  klass->realize = gdk_gl_context_real_realize;
+
   /**
    * GdkGLContext:display:
    *