Empty/non-working static library

Compiling with meson --default-library=static results in a practically empty static library, both in Archlinux GCC and mingw-w64 GCC.

With https://github.com/mesonbuild/meson/pull/3939 patched into meson, and with a seemingly more complete static library, there's still issues when using libdav1d with another project (specifically, https://patchwork.ffmpeg.org/patch/10512/).

Only the shared library seems to result in a working library.

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