Equivalent functionality (allowing dependencies to be meson subprojects
and fixing the zlib HAVE_UNISTD_H issue) was merged upstream now
(thanks kasper), so this is no longer necessary. dav1d is now
enabled using -Dffmpeg:libdav1d=enabled.
Also bump libdav1d and turn the wrap patches into diffs. This requires
bumping the minimum meson version to 0.63, but meson 1.0 is available
pretty much everywhere now so this should be fine.