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.

GNOME/librsvg#328
3 jobs for <span class="ref-name">masking-endianness</span> in 8 minutes and 4 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #93853
fedora test

00:07:42

 
  Lint
manual #93855
allowed to fail manual
clippy
passed #93854
rustfmt

00:00:22