Use meson to build pixman and use a64-neon=disabled where needed

Autotools build isn't supported any more (see https://gitlab.freedesktop.org/pixman/pixman/-/issues/79) so let's use meson. Except "unstable" modules, which use the latest git version that supports Apple processors, a64-neon should be disabled for the 0.42.2 where the support hasn't been added yet.

I was thinking about possibly porting https://gitlab.freedesktop.org/pixman/pixman/-/commit/67490a8bc12647a78d92f62f0af50cd7f4146ca1 for the 0.42.2 branch, but at least for Geany (where I tested it on scrolling text for about 4 seconds), I got only a minor improvement with the assembly version.

This profile comes from the 0.42.2 branch without the assembly optimizations:

and this one from the current git master with the optimizations enabled:

The difference is just minor unfortunately.