Add cogl_primitive_foreach_attribute()

This adds a public function to iterate the attributes of a
CoglPrimitive. Previously there was no way to query back the
attributes but there was methods to query back all of the other
properties.

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

doc/reference/cogl-2.0-experimental/cogl-2.0-experimental-sections.txt

@@ -238,6 +238,8 @@ cogl_primitive_set_attributes
 cogl_primitive_set_indices
 cogl_primitive_draw
 cogl_primitive_copy
+CoglPrimitiveAttributeCallback
+cogl_primitive_foreach_attribute
 </SECTION>
 
 <SECTION>