[cairo] _cairo_clip_intersect_mask should set the clip mode

[Jonathan Watt]

Hi,

I've just been debugging an SVG bug in Mozilla [1] and discovered that the
source of the problem is _cairo_clip_intersect_mask failing to set the 'mode'
member on the new clip to CAIRO_CLIP_MODE_MASK. Seems like this should happen as
the first line after 'DONE:'.

Regards,
Jonathan

1. https://bugzilla.mozilla.org/show_bug.cgi?id=409227