[cairo] Transparent images in cairo_win32_printing_surface_t have a black background

[Rodrigo Rivas]

Hi!

A few weeks ago I sent a bug report[1] to the Bugzilla, but it didn't
attract enough attention.

The patch itself is just one line initializing an uninitialized variable.

Thanks.

[1]: https://bugs.freedesktop.org/show_bug.cgi?id=91835