mask: move bit manipulation to the Pixel object

Pixel already encapsulates (un)premultiply etc, so let's keep
all the the bit fiddling in one place.
7 jobs from parse in 10 minutes and 17 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #177597
fedora test

00:08:50

 
  Lint
manual #177599
allowed to fail manual
clippy
passed #177598
rustfmt

00:00:46

failed #177600
tarball

00:01:26

 
  Cross Distro
skipped #177603
debian_i386:testing
skipped #177602
debian:testing
skipped #177601
fedora:rawhide
 
Name Stage Failure
failed
tarball Lint
  No space left on device (os error 28)
make[3]: Leaving directory '/builds/pborelli/librsvg/_build'
make[3]: *** [Makefile:1948: dist-hook] Error 101
make[2]: *** [Makefile:1567: distdir-am] Error 2
make[2]: Leaving directory '/builds/pborelli/librsvg/_build'
make[1]: *** [Makefile:1560: distdir] Error 2
make[1]: Leaving directory '/builds/pborelli/librsvg/_build'
make: *** [Makefile:1664: dist] Error 2
ERROR: Job failed: exit code 1