ffmpeg-headless: enable additional encoders by default

This adds some extremely helpful and popular encoders in by default:
* openjpeg
* celt
* libwebp
* libaom

On the `master` branch, closure size for ffmpeg-headless went up 18.5 MiB.
```
$ nix store diff-closures nixpkgs#ffmpeg-headless^bin .#ffmpeg-headless^bin
celt: ∅ → 0.11.3, +168.4 KiB
ffmpeg-headless: +70.0 KiB
giflib: ∅ → 5.2.2, +398.7 KiB
lcms2: ∅ → 2.16, +466.2 KiB
lerc: ∅ → 4.0.0, +840.2 KiB
libaom: ∅ → 3.9.0, +8047.8 KiB
libdeflate: ∅ → 1.20, +427.0 KiB
libtiff: ∅ → 4.6.0, +655.9 KiB
libvmaf: ∅ → 3.0.0, +2665.0 KiB
libwebp: ∅ → 1.4.0, +2559.7 KiB
openjpeg: ∅ → 2.5.2, +1525.1 KiB
zstd: ∅ → 1.5.6, +1158.0 KiB

$ nvd diff $(nix build nixpkgs#ffmpeg-headless^bin --print-out-paths --no-link) $(nix build .#ffmpeg-headless^bin --print-out-paths --no-link)
<<< /nix/store/4n60lnj3zkjpasd4c56bzhpx2m8lc1sx-ffmpeg-headless-6.1.1-bin
>>> /nix/store/884f487w5hac6rs94jq6hq5zqkxdv666-ffmpeg-headless-6.1.1-bin
Added packages:
[A.]  #1  celt        0.11.3
[A.]  #2  giflib      5.2.2
[A.]  #3  lcms2       2.16
[A.]  #4  lerc        4.0.0
[A.]  #5  libaom      3.9.0
[A.]  #6  libdeflate  1.20
[A.]  NixOS#7  libtiff     4.6.0
[A.]  NixOS#8  libvmaf     3.0.0
[A.]  NixOS#9  libwebp     1.4.0 x2
[A.]  NixOS#10  openjpeg    2.5.2
[A.]  NixOS#11  zstd        1.5.6
Closure size: 66 -> 78 (15 paths added, 3 paths removed, delta +12, disk usage +18.5MiB).
```

pkgs/development/libraries/ffmpeg/generic.nix

@@ -34,7 +34,7 @@
 
   # Feature flags
 , withAlsa ? withHeadlessDeps && stdenv.isLinux # Alsa in/output supporT
-, withAom ? withFullDeps # AV1 reference encoder
+, withAom ? withHeadlessDeps # AV1 reference encoder
 , withAppKit ? withHeadlessDeps && stdenv.isDarwin # Apple AppKit framework
 , withAribcaption ? withFullDeps && lib.versionAtLeast version "6.1" # ARIB STD-B24 Caption Decoder/Renderer
 , withAss ? withHeadlessDeps && stdenv.hostPlatform == stdenv.buildPlatform # (Advanced) SubStation Alpha subtitle rendering
@@ -45,7 +45,7 @@
 , withBs2b ? withFullDeps # bs2b DSP library
 , withBzlib ? withHeadlessDeps
 , withCaca ? withFullDeps # Textual display (ASCII art)
-, withCelt ? withFullDeps # CELT decoder
+, withCelt ? withHeadlessDeps # CELT decoder
 , withChromaprint ? withFullDeps # Audio fingerprinting
 , withCodec2 ? withFullDeps # codec2 en/decoding
 , withCoreImage ? withHeadlessDeps && stdenv.isDarwin # Apple CoreImage framework
@@ -88,7 +88,7 @@
 , withOpencoreAmrwb ? withFullDeps && withVersion3 # AMR-WB decoder
 , withOpengl ? withFullDeps && !stdenv.isDarwin # OpenGL rendering
 , withOpenh264 ? withFullDeps # H.264/AVC encoder
-, withOpenjpeg ? withFullDeps # JPEG 2000 de/encoder
+, withOpenjpeg ? withHeadlessDeps # JPEG 2000 de/encoder
 , withOpenmpt ? withFullDeps # Tracked music files decoder
 , withOpus ? withHeadlessDeps # Opus de/encoder
 , withPlacebo ? withFullDeps && !stdenv.isDarwin # libplacebo video processing library
@@ -121,7 +121,7 @@
 , withVpl ? false # Hardware acceleration via intel libvpl
 , withVpx ? withHeadlessDeps && stdenv.buildPlatform == stdenv.hostPlatform # VP8 & VP9 de/encoding
 , withVulkan ? withSmallDeps && !stdenv.isDarwin
-, withWebp ? withFullDeps # WebP encoder
+, withWebp ? withHeadlessDeps # WebP encoder
 , withX264 ? withHeadlessDeps && withGPL # H.264/AVC encoder
 , withX265 ? withHeadlessDeps && withGPL # H.265/HEVC encoder
 , withXavs ? withFullDeps && withGPL # AVS encoder