gitlab#328 - Make masking work on big-endian
gitlab#328 - Make masking work on big-endian

The code only worked in little-endian machines; it wrote to the pixels
directly.  Now it uses the pixel iterator for SharedImageSurface,
which handles endianness properly.

https://gitlab.gnome.org/GNOME/librsvg/issues/328

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg