[julia-users] Adding another function to Cairo.jl

[cormullion]

I noticed that Cairo offers a function called `cairo_paint_with_alpha`, 
that accompanies `cairo_paint`, 
(http://cairographics.org/manual/cairo-cairo-t.html#cairo-paint).

At present Julia's Cairo.jl doesn't seem to have this defined. Is it 
possible to test it out by defining it using `ccall`, but without adding it 
to Cairo.jl? I was thinking something like:

    using Cairo

    function paint_with_alpha(ctx::CairoContext, a = 0.5)
        ccall((:cairo_paint_with_alpha, _jl_libcairo), Ptr{Void},
                (Ptr{Void}, Float64),
                ctx.ptr, a)  
    end

which doesn't work, obviously, cos I'm just guessing at the syntax...

If it works, it could be added to a pull request.