[cairo] Quartz gradients consistency

[Andrea Canciani]

After some discussion with Robert O'Callahan and Benjamin Otte, I've written
down a solutions to fix quartz gradients that should not break Mozilla nor GTK.

Moreover it should make gradients consistent between all the operations
(paint, fill, stroke, show_glyphs and (NYI, but soon) mask) and independent
of clip and path extents.

For "new" surfaces the gradient extents are the surface extents, but for
surfaces created with a relation to another surface (cairo_push/pop_group,
cairo_surface_create_similar) it makes the gradient extents be the same
as the ones of the original surface (and keeps this consistency even when
the surfaces are "moved" with cairo_surface_set_device_offset).
This means that both clip+draw and push+draw+pop should work as expected.

Any suggestions on how the attached patch could be improved?
Andrea
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