[Cairo] Suggestion for replacing cairo_copy with set_gstate.

[Banlu Kemiyatorn]

Dear Cairo users and developers,
    I'm currently using cairo_copy for gstates' stack management without 
being able to
take fully advantage of cairo_save and cairo_restore because it lacks of 
set_gstate.
cairo_copy by itself is obscure, since it cannot hint user that it 
didn't copy the gstate stack.
Changing to cairo_fork,snap,etc would need addition documentation and it 
is incongruent
with the current gstate API so I suggest to remove cairo_copy and expose 
gstate_t instead.
This should be more consistent with PostScript and cleaner API in my 
opinion.
Best regards,
id