contrib: dav1d: update to 1.2.1

- Fix a threading race on task_thread.init_done
- NEON z2 8bpc and high bit-depth optimizations
- SSSE3 z2 high bit-depth optimziations
- Fix a desynced luma/chroma planes issue with Film Grain
- Reduce memory consumption
- Improve dav1d_parse_sequence_header() speed
- OBU: Improve header parsing and fix potential overflows
- OBU: Improve ITU-T T.35 parsing speed
- Misc buildsystems, CI and headers fixes

Source:

https://code.videolan.org/videolan/dav1d/-/releases/1.2.1
(cherry picked from commit 2b9c1fdf)

contrib/src/dav1d/SHA512SUMS

-a3a7e162e45181449cd42af3a4d36669a850a4ee9ab17641dcd63d84406444566e8ebc7caa55b0620ab581039f36d19a90218a40f52ebbe525b37ed9493fb3f3  dav1d-1.0.0.tar.xz
+f172eebc0a5f6c51d31fc9e9758c2dd0de51d8a5d0e00c93a5f2b1b16b7b4a37b365f9c56dea95d400e66b63af5fa4c63d9e720719ac38852777fc8c6066e4a7  dav1d-1.2.1.tar.xz

contrib/src/dav1d/rules.mak

 # libdav1d
 
-DAV1D_VERSION := 1.0.0
+DAV1D_VERSION := 1.2.1
 DAV1D_URL := $(VIDEOLAN)/dav1d/$(DAV1D_VERSION)/dav1d-$(DAV1D_VERSION).tar.xz
 
 PKGS += dav1d