meson: remove pixman dependency

AFAICT from the git history, what happened is that the gtk demos rely on gtk being built with pixman support. pkg-config isn't really expressive enough to have that information, so the solution that was come up with was to search for pixman as well as gtk+ and hope that pixman being installed was.

This isn't actually used anywhere in the meson build anyway, and it's causing problems for projects that want to use pixman as a supproject (there's a port of cairo underway that's hitting this), because it confuses meson.