Meson build system

This adds a meson build system for pixman. It's got most of the features of a meson build, more details are in the commit message for the patch adding meson support. I've also thrown in a .editorconfig that sets the meson settings to be correct, and sets tabs to be the correct size elsewhere.

I've tested this with x86 and x86_64 on Linux and Windows (using mingw), and I've cross compiled to Aarch64. There are a few tests that fail on Windows, I'm not really sure why. I haven't tested this on any other architectures, so testing on those would be appreciated.

The one thing that is notably different about this meson build system is support for very old OSes and compilers. Meson itself does not support SunCC (which AFAIU is deprecated), this build system also assumes that you're not running on Solaris 9 (which was EOL'd in 2014) and that you have at least GCC 4.x or an equivalently modern Clang. It also takes steps not to test compile arch specific optimizations if compiling for a different arch (say SSE2 when compiling for Aarch64).