Bug 783528 - PDF export ignores layer masks.

Cairo doc is pretty clear about cairo_mask_surface():
> A drawing operator that paints the current source using the alpha
> channel of @surface as a mask
Therefore when creating a mask surface, it must be in one of
CAIRO_FORMAT_* with an alpha channel and the mask data must be copied
into this alpha channel. I chose CAIRO_FORMAT_A8 (a format with alpha
only) which I map to "Y u8" babl format in GEGL so that
gegl_buffer_copy() copies the contents of Y into A.