[cairo] Resize or extend a cairo surface

[Benjamin Warren]

The situation is that my python script is rendering svg images to a
cairo.Context, which is attached to a cairo.ImageSurface with set
dimensions. When the script 'runs out of space' to draw the svg images
I want it to effectively resize/extend the surface dimensions to make
space for more svg images.

(I cannot preallocate the space because I don't know the number or
size of the images in advance.)