x265: build a single shared library for all bit-depths

[dominikh]

This builds the 10-bit and 12-bit versions of x265 as static
libraries, which then get linked into the 8-bit dynamic library and
executable. This causes x265 to default to 8-bit, but make 10- and
12-bit available to callers that use introspection, such as ffmpeg.

$ x265 -V
x265 [info]: HEVC encoder version 3.2
x265 [info]: build info [Linux][GCC 9.3.0][64 bit] 8bit+10bit+12bit
x265 [info]: using cpu capabilities: MMX2 SSE2Fast LZCNT SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2

$ ffmpeg -h encoder=libx265
[...]
    Supported pixel formats: yuv420p yuvj420p yuv422p yuvj422p yuv444p yuvj444p gbrp yuv420p10le yuv422p10le yuv444p10le gbrp10le yuv420p12le yuv422p12le yuv444p12le gbrp12le gray gray10le gray12le

Inspired by @codyopel's comment on #80405.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[mdlayher]

/marvin opt-in

[mdlayher]

For anyone reading, I'm running nixpkgs-review pr 92052 locally but it's taking a very long time to pull all the reverse dependencies and such. I'll share another update when it's done.

[mdlayher]

I let it run for about 3 hours and it noted 28 package failures before starting compilation on really big packages like Blender. I don't think my desktop is beefy enough to run this whole workload: https://gist.github.com/mdlayher/7e4cc832f5cc0f2ec7e6934bd74a8c0f

[mdlayher]

I can confirm that this also works locally on my machine, but I'm out of my depth as far as diagnosing any possible reverse dependency issues:

✔ ~/src/github.com/nixos/nixpkgs [:bdd65f3f69d|✔] 
14:21 $ nix run -f default.nix x265
[3 built, 5 copied (34.5 MiB), 7.9 MiB DL]
✔ ~/src/github.com/nixos/nixpkgs [:bdd65f3f69d|✔] 
14:25 $ nix-build -A x265
/nix/store/16lp8nyar1rwx7iz0flvk0kybcnnwwp0-x265-3.2
✘-1 ~/src/github.com/nixos/nixpkgs [:bdd65f3f69d|✔] 
14:25 $ ./result/bin/x265 -V
x265 [info]: HEVC encoder version 3.2
x265 [info]: build info [Linux][GCC 9.3.0][64 bit] 8bit+10bit+12bit
x265 [info]: using cpu capabilities: MMX2 SSE2Fast LZCNT SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2