[cairo] Set opacity for surface

[Jerónimo Barraco]

Hi everyone, i've been working on a software wich is some kind of  
abstraction on top of cairo, and i'm having a problem.
I'm using objects that extends vectors (there's no inheritance) and have  
some methods for specifying how to stroke and fill thems.
Using solids and patterns is ok, but when i need to use a texture (a  
surface loaded from a PNG) i can't find a way to specify an alpha.
i'm using "ctx.set_surface" to assign a surface, and using "ctx.fill" or  
"ctx.stroke_preserve" acordingly.

I'm setting the source in one part of my code (using interchangeable  
methods for different "predesigned" effects) and calling
fill, stroke_preserve (and a hand-made shadow) in another part of the code.

so i need a solution that doesnt affect the other functions (rgba and  
patterns), wont require me to change where i use stroke/fill,
and is as fast as posible, as i need to redraw it a lot of times, (and  
simulating a opacity animation)

Thanks in advance..

here's the code:

here are the methods that set a source:
Line 13 is where i use a "texture" (png surface wich will change from  
object to object and part of the object), and i need a random opacity that  
will probably will be used once.
http://www.assembla.com/code/kafx/subversion/nodes/branches/Implementacion/Cairo/libs/cairo_basic.py

here is where i call the methods from above:
Line 428 is the method that will call them.
http://www.assembla.com/code/kafx/subversion/nodes/branches/Implementacion/Cairo/libs/cairo_extra.py
notice lines 458, 462 and 472, those __S* methods are pointers to one of  
the methods in the file from above, that's why i need them to be  
interchangeable
for using with stroke and fill.

i only came up with the idea of creating a new surface, create a mask with  
the opacity, paint the source using the mask, and reusing the result, but  
that will be TOO costly.
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