Bug 918 - Group doesn't clip if it's children are clipped

* clutter/cogl/common/cogl-clip-stack.h: 
	* clutter/cogl/common/cogl-clip-stack.c: Added functions to
	maintain a stack of clipping rectangles.

	* clutter/cogl/gles/cogl.c: 
	* clutter/cogl/gl/cogl.c: The cogl_clip_set and unset functions
	have moved into cogl-clip-stack.c which calls back to cogl.c to
	set the actual rectangles. Multiple clip rectangles are combined
	by merging the stencil buffers.

	* clutter/cogl/gles/cogl-primitives.c (_cogl_path_fill_nodes): 
	* clutter/cogl/gl/cogl-primitives.c (_cogl_path_fill_nodes): Merge
	the stencil buffer with the contents of the clipping stack after
	drawing the path.

	* clutter/cogl/gles/cogl-context.h (CoglContext): 
	* clutter/cogl/gl/cogl-context.h (CoglContext): Store the number
	of available stencil bits.

	* clutter/cogl/common/Makefile.am
	(libclutter_cogl_common_la_SOURCES): Added cogl-clip-stack.c

common/cogl-clip-stack.h

@@ -0,0 +1,37 @@
+/*
+ * Clutter COGL
+ *
+ * A basic GL/GLES Abstraction/Utility Layer
+ *
+ * Authored By Matthew Allum  <mallum@openedhand.com>
+ *
+ * Copyright (C) 2008 OpenedHand
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#ifndef __COGL_CLIP_STACK_H
+#define __COGL_CLIP_STACK_H
+
+void cogl_clip_set (ClutterFixed x_offset,
+		    ClutterFixed y_offset,
+		    ClutterFixed width,
+		    ClutterFixed height);
+void cogl_clip_unset (void);
+void _cogl_clip_stack_rebuild (gboolean just_stencil);
+void _cogl_clip_stack_merge (void);
+
+#endif /* __COGL_CLIP_STACK_H */