meson: Add a meson build system

This commit adds a meson build system for pixman. It carries the usual
improvements of meson, better clean build time, much better incremental
build times, while being simpler and easier to understand.

This takes advantage of some features from the most recent versions of
meson: the builtin openmp dependency and the feature option type.

There are a couple of things that I've done a bit differently than the
autotools build system, I've built a libdemos which is the utilities
from the demos folder, and I've linked the demos with libtestutils from
tetsts, otherwise I expect that most things will be the same.

I've tested so far cross compiling from x86_64 -> x86, x86_64 ->
Aarch64, and Linux to Windows via mingw, as well as native x86_64 Linux
builds which all work. I've also built with mingw nativly, there are
some test failures there. An MSVC build can be generated, but fails.

v2: - set WORDS_BIGENDIAN in the config for big endian systems.