[cairo] Extend methods for gradient patterns

[Torsten Schoenfeld]

According to <http://cairographics.org/manual/bindings-patterns.html>, 
cairo_pattern_set_extend() and cairo_pattern_get_extend() should be 
methods of the surface pattern type.  In particular, they should not be 
available to gradient patterns.  But snooping around cairo's source a 
bit, it does seem to be sensible to use the extend stuff on gradient 
patterns at least.  Who's right?