JPEG XL decoding support

[p0nce]

• [x] Some performance optimization probably useful, decode is too slow => let's wait if anyone complains
• [x] Integrate j40.h translation, proven to work
• [x] Implement JPEG XL detection, annoying because it is inside ISOBMFF (EDIT: actually no, we uspport only raw bitstream, there exists other JPEG XL that we don't support! with ISO BMFF EXIF and boxes and all)
• [x] Check for leaks => ok, those were "fixed"
• [x] Is it possible/easy to get alpha support? => difficult without the spec, which I don't want to pay for
• [x] Measure cost in binary kb => 190kb binary in x86_64. Build time: about 30ms according to redub.
• [x] Measure decoding time of JXL vs decoding time of JPEG, at equivalent quality => A 200kb JPEG XL decodes in 218.73 ms while the equivalent quality JPEG decode in 21.33. 10x slower on decode as of Apr 18th
• [x] Memory pressure: the equivalent JPEG can decode in 2.79mb of allocations. The same quality JPEG XL uses 23mb of malloc/realloc. Which seems acceptable.

[p0nce]

• [x] Intel Inspector report this 48-byte leak (EDIT: confirmed)

• [x] Also a whole plane being leaked

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• [ ] Now more fishy: an "invalid memcpy argument" here:

• [ ] and another one in:

but not sure what constraint is broken for those 2.

[p0nce]

First 48 byte leak: in j40__combine_vardct, the m.channel that were allocated in case J40__TR_PALETTE is overwritten without mercy. Also leaking a plane used for palette before. I have no idea what is happening. Anyway, the leaks are "fixed". 4def529f522ba468d2f142a04fb5db373503286e