[cairo] Public API for pattern duplication

[Mark Vender]

Hello,

Cairo currently has no public API for pattern duplication. This 
functionality would simplify usage of cairo in the case when there's a 
need to use a modified pattern for drawing, but the given pattern itself 
must not be modified.

The function for pattern copying is included in the TODO list as 
`cairo_pattern_copy`. It seems that the implementation is already in 
master as `_cairo_pattern_create_copy`, so probably only a wrapper 
around that function is needed.

     Thanks,
     Mark