gdk_gl_pixmap_make_current() validates the "context" argument using
GDK_IS_GL_PIXMAP(), when it should be using GDK_IS_GL_CONTEXT().
Consequently, the function always fails.

Fix:

Index: gtkgl/gdkgl.c
===================================================================
RCS file: /cvs/gnome/gtkglarea/gtkgl/gdkgl.c,v
retrieving revision 1.12
diff -u -r1.12 gdkgl.c
--- gtkgl/gdkgl.c	9 Jun 2002 16:22:36 -0000	1.12
+++ gtkgl/gdkgl.c	29 Aug 2004 03:56:20 -0000
@@ -395,7 +395,7 @@
   GLXContext glxcontext;
 
   g_return_val_if_fail(GDK_IS_GL_PIXMAP(glpixmap), FALSE);
-  g_return_val_if_fail(GDK_IS_GL_PIXMAP(context), FALSE);
+  g_return_val_if_fail(GDK_IS_GL_CONTEXT(context), FALSE);
 
   dpy        = context->xdisplay;
   glxpixmap  = glpixmap->glxpixmap;