Add the missing cast macros for some buffer objects

CoglPixelBuffer, CoglAttributeBuffer and CoglIndexBuffer were missing
public cast macros.

Reviewed-by: Robert Bragg <robert@linux.intel.com>

cogl/cogl-index-buffer.h

@@ -43,6 +43,8 @@ G_BEGIN_DECLS
  * FIXME
  */
 
+#define COGL_INDEX_BUFFER(buffer) ((CoglIndexBuffer*) buffer)
+
 typedef struct _CoglIndexBuffer	      CoglIndexBuffer;
 
 /**