image/jpeg: add support for non-standard chroma subsampling ratios

Add "flex mode" decoding for JPEG images with non-standard YCbCr
subsampling ratios that do not match the predefined YCbCrSubsampleRatio
values. This includes cases where:
1. Cb and Cr components have different sampling factors
2. The Y component does not have the maximum sampling factors

Such images were previously rejected with
"unsupported luma/chroma subsampling ratio"
but should be valid according to the JPEG specification:
https://www.w3.org/Graphics/JPEG/itu-t81.pdf

Flex mode allocates a YCbCr444 backing buffer and manually expands
pixels according to each component's sampling factors relative to the
maximum. This approach mirrors the implementation in kovidgoyal/imaging.

Fixes #2362

goos: darwin
goarch: arm64
pkg: image/jpeg
cpu: Apple M4 Max
                     │   old.txt   │               new.txt               │
                     │   sec/op    │    sec/op     vs base               │
FDCT-16                576.9n ± 1%   578.9n ±  1%       ~ (p=0.565 n=10)
IDCT-16                550.1n ± 0%   573.6n ±  3%  +4.27% (p=0.000 n=10)
DecodeBaseline-16      520.6µ ± 4%   523.8µ ±  2%       ~ (p=0.796 n=10)
DecodeProgressive-16   767.9µ ± 3%   747.0µ ± 10%       ~ (p=0.123 n=10)
EncodeRGBA-16          7.869m ± 3%   8.485m ±  6%  +7.82% (p=0.001 n=10)
EncodeYCbCr-16         8.761m ± 6%   8.021m ±  2%  -8.45% (p=0.001 n=10)
geomean                143.5µ        143.8µ        +0.18%

                     │   old.txt    │               new.txt                │
                     │     B/s      │      B/s       vs base               │
DecodeBaseline-16      113.2Mi ± 4%   112.5Mi ±  2%       ~ (p=0.796 n=10)
DecodeProgressive-16   76.75Mi ± 3%   78.90Mi ± 10%       ~ (p=0.123 n=10)
EncodeRGBA-16          148.9Mi ± 3%   138.1Mi ±  7%  -7.25% (p=0.001 n=10)
EncodeYCbCr-16         100.3Mi ± 7%   109.6Mi ±  2%  +9.23% (p=0.001 n=10)
geomean                106.7Mi        107.7Mi        +0.86%

                     │   old.txt    │                new.txt                │
                     │     B/op     │     B/op      vs base                 │
DecodeBaseline-16      61.55Ki ± 0%   61.55Ki ± 0%       ~ (p=1.000 n=10) ¹
DecodeProgressive-16   253.6Ki ± 0%   253.6Ki ± 0%       ~ (p=0.124 n=10)
EncodeRGBA-16          4.438Ki ± 0%   4.438Ki ± 0%       ~ (p=1.000 n=10) ¹
EncodeYCbCr-16         4.438Ki ± 0%   4.438Ki ± 0%       ~ (p=1.000 n=10) ¹
geomean                23.55Ki        23.55Ki       +0.00%
¹ all samples are equal

                     │  old.txt   │               new.txt               │
                     │ allocs/op  │ allocs/op   vs base                 │
DecodeBaseline-16      5.000 ± 0%   5.000 ± 0%       ~ (p=1.000 n=10) ¹
DecodeProgressive-16   13.00 ± 0%   13.00 ± 0%       ~ (p=1.000 n=10) ¹
EncodeRGBA-16          7.000 ± 0%   7.000 ± 0%       ~ (p=1.000 n=10) ¹
EncodeYCbCr-16         7.000 ± 0%   7.000 ± 0%       ~ (p=1.000 n=10) ¹
geomean                7.512        7.512       +0.00%
¹ all samples are equal

Co-authored-by: Kovid Goyal <kovidgoyal@gmail.com>

Change-Id: Ic7353ce6a0b229cb6aa775bb05044d6bcded7ab2
Reviewed-on: https://go-review.googlesource.com/c/go/+/738280
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
Reviewed-by: Nigel Tao <nigeltao@google.com>

0 files changed, 0 insertions, 0 deletions