Most of the code was taken from the print plugin, and it includes iterating
over pixel regions (instead of fetching them at once), correct support for RGBA
(which was buggy previously), etc.
This commit removes the usage of A8 cairo surfaces for GRAY layers, simply
because it seems not to work. This should be investigated and fixed in order
to optimize the file size (by saving gray images as gray).
Also should be checked is why we treat the cairo bpp as 4 when calculating the
pixel locations in RGB cairo buffers. The print plug-in does this and it works,
and when I tried 3 it didn't work. May be because of word alignment...