SIMD Encoder : an AVX2 implementation

[MKCG]

Hello,
this pull request implements an AVX2 based encoder.

It also introduces a new structure named qoi_encoder_t that could be use to implement a streaming encoder.

When encoding the images provided on the official website, I get those results :

branch	type	encode ms	encode mpps	branches	branches misses
master	any	14.4	157.28	28 137 276 765	1 492 397 899
this pull request	any	7.9	284.32	11 348 182 313	714 559 161
master	RGB	40.0	132.31	21 054 094 125	1 322 613 445
this pull request	RGB	23.0	229.90	9 418 298 665	633 968 387
master	RGBA	3.4	281.22	7 080 735 865	172 313 079
this pull request before 2672a47	RGBA	1.8	524.59	1 834 707 343	79 617 576
this pull request after 2672a47	RGBA	1.5	648.99	1 880 977 270	79 359 904

Note: 2672a47 avoids performing cpu intensive operations when the next 8 pixels belong to the same run. Only images with flat surfaces like screenshots will benefit from it.

Note: RGB images are a bit tricky to encode using AVX2 instructions, and this implementation can still be improved.

[MKCG]

My PHP library use a C library defining a chunk based encoder to lower the memory footprint : https://github.com/MKCG/php-qoi/blob/main/src/FFI/lib/qoi.c#L112

I will probably open a pull request here to implement a streaming encoder.

[phoboslab]

This is very cool, but I'm sorry to say that I will not merge it. I want this "reference" encoder here to stay as simple as possible. Also, I neither have the expertise nor the desire to maintain this SIMD implementation.

If you publish your encoder under a different name (fastqoi? rapidqoi? simdqoi?) I will happily mention it in the readme here!

[MKCG]

Actually I add little hope that it would be accepted since I already knew you weren't very familiar with AVX2 instructions from a previous comment on the final specification thread.

"simdqoi" would work for me, that would be a nice tribute to the amazing work of Daniel Lemire, Geoff Langdale and so many others. I guess that now I also have to make an AVX2 based decoder before publishing it.

Thanks for your time.