Right now, we assume the stride for the image surface needs to
be 4 byte aligned. This is, in fact, true, but it's better to
ask cairo for the alignment requirement directly rather than
assume we know the alignment rules.
This commit changes the code to use cairo_format_stride_for_width
to calculate a suitable rowstride for pixman.
https://bugzilla.gnome.org/show_bug.cgi?id=760897
1e001eaa78