[backport] contrib: dav1d: update to 1.5.1

Previously discussed in !6723 (closed), we decided at the technical meeting to push ahead with dav1d 1.5.1 as the benefits of having more optimal paths for newer devices outweighed the tradeoff of dropping SSE2 optimizations (as if you're limited to SSE2 you're probably already going to have a hard time decoding AV1 so it doesn't make sense to pin all 3.0 users to the lowest common denominator).

Changes for 1.5.1 'Sonic':

1.5.1 is a minor release of dav1d, focusing on optimizations and stack reduction:

• Rewrite of the looprestoration (SGR, wiener) to reduce stack usage
• Rewrite of {put,prep}_scaled functions

Now, the required stack space for dav1d should be: 62 KB on x86_64 and 58KB on arm and aarch64.

• Improvements on the SSSE3 SGR
• Improvements on ARM32/ARM64 looprestoration optimizations
• RISC-V: blend optimizations for high bitdepth
• Power9: blend optimizations for 8bpc
• Port RISC-V to POSIX/non-Linux OS
• AArch64: Add Neon implementation of load_tmvs
• Fix a rare, but possible deadlock, in flush()

Changes for 1.5.0 'Sonic':

1.5.0 is a major release of dav1d, that:

• WARNING: we removed some of the SSE2 optimizations, so if you care about systems without SSSE3, you should be careful when updating!
• Add Arm OpenBSD run-time CPU feature
• Optimize index offset calculations for decode_coefs
• picture: copy HDR10+ and T35 metadata only to visible frames
• SSSE3 new optimizations for 6-tap (8bit and hbd)
• AArch64/SVE: Add HBD subpel filters using 128-bit SVE2
• AArch64: Add USMMLA implempentation for 6-tap H/HV
• AArch64: Optimize Armv8.0 NEON for HBD horizontal filters and 6-tap filters
• Power9: Optimized ITX till 16x4.
• Loongarch: numerous optimizations
• RISC-V optimizations for pal, cdef_filter, ipred, mc_blend, mc_bdir, itx
• Allow playing videos in full-screen mode in dav1dplay