build: Add variables to gdkpixbuf_dep to match pkg-config

If gdk-pixbuf is built as a fallback subproject, the superproject's gdkpixbuf_dep.get_variable() won't read from pkg-config as usual, but instead will read from variables declared in gdkpixbuf_dep. We can't easily export paths to binaries in that case -- do we use the built path or the installed one? -- but we already call meson.override_find_program(), so the superproject can just use find_program() for these. Export all other pkg-config variables in gdkpixbuf_dep.