image/jpeg: replace fdct.go and idct.go with new implementation in dct.go

The fdct.go and idct.go files were derived from the MPEG-SSG and
JPEG-IJG reference code and therefore carry licenses specific to those
groups. Various license checkers flag these files as potentially
problematic. The code is also not terribly well documented.

This CL fixes the license problem by adding a new, from-scratch
implementation using a different algorithm. As a bonus, the new code
is both faster and more accurate than the old encumbered code.

On speed, the new code is up to 20% faster; benchmarks below.

On accuracy, in the set of blocks used in the test, we can measure
the number of output values that are off-by-one from the exact rounded answer.
The old FDCT was off in 8.6% of values; the new one is off in 2.5%.
The old IDCT was off in 1.4% of values; the new one is off in 1.2%.

goos: darwin
goarch: arm64
pkg: image/jpeg
cpu: Apple M3 Pro
                     │     old     │                 new                 │
                     │   sec/op    │   sec/op     vs base                │
FDCT-12                619.6n ± 3%   586.5n ± 1%   -5.34% (p=0.000 n=10)
IDCT-12                752.4n ± 4%   628.0n ± 1%  -16.54% (p=0.000 n=10)

goos: linux
goarch: amd64
cpu: Intel(R) Xeon(R) CPU @ 2.30GHz
               │     old     │                 new                 │
               │   sec/op    │   sec/op     vs base                │
FDCT-16          1.817µ ± 0%   1.542µ ± 0%  -15.11% (p=0.000 n=10)
IDCT-16          1.897µ ± 0%   1.514µ ± 0%  -20.22% (p=0.000 n=10)

goos: linux
goarch: arm64
cpu: whatever gotip-linux-arm64 has
              │     old     │                new                 │
              │   sec/op    │   sec/op     vs base               │
FDCT-8          1.844µ ± 0%   1.847µ ± 0%  +0.14% (p=0.000 n=10)
IDCT-8          2.127µ ± 0%   1.973µ ± 0%  -7.26% (p=0.000 n=10)

Change-Id: Ie6d14103c8478ba5a779f234da84f345828eb925
Reviewed-on: https://go-review.googlesource.com/c/go/+/705518
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Auto-Submit: Russ Cox <[email protected]>
Reviewed-by: Nigel Tao <[email protected]>

Author: rsc

src/image/decode_example_test.go

@@ -70,22 +70,22 @@ func Example() {
 	}
 	// Output:
 	// bin               red  green   blue  alpha
-	// 0x0000-0x0fff:    364    790   7242      0
-	// 0x1000-0x1fff:    645   2967   1039      0
-	// 0x2000-0x2fff:   1072   2299    979      0
-	// 0x3000-0x3fff:    820   2266    980      0
-	// 0x4000-0x4fff:    537   1305    541      0
-	// 0x5000-0x5fff:    319    962    261      0
-	// 0x6000-0x6fff:    322    375    177      0
-	// 0x7000-0x7fff:    601    279    214      0
-	// 0x8000-0x8fff:   3478    227    273      0
-	// 0x9000-0x9fff:   2260    234    329      0
-	// 0xa000-0xafff:    921    282    373      0
-	// 0xb000-0xbfff:    321    335    397      0
-	// 0xc000-0xcfff:    229    388    298      0
-	// 0xd000-0xdfff:    260    414    277      0
-	// 0xe000-0xefff:    516    428    298      0
-	// 0xf000-0xffff:   2785   1899   1772  15450
+	// 0x0000-0x0fff:    362    793   7245      0
+	// 0x1000-0x1fff:    648   2963   1036      0
+	// 0x2000-0x2fff:   1072   2301    977      0
+	// 0x3000-0x3fff:    819   2266    982      0
+	// 0x4000-0x4fff:    537   1303    541      0
+	// 0x5000-0x5fff:    321    964    261      0
+	// 0x6000-0x6fff:    321    375    177      0
+	// 0x7000-0x7fff:    599    278    213      0
+	// 0x8000-0x8fff:   3478    228    275      0
+	// 0x9000-0x9fff:   2260    233    328      0
+	// 0xa000-0xafff:    921    282    374      0
+	// 0xb000-0xbfff:    322    335    395      0
+	// 0xc000-0xcfff:    228    388    299      0
+	// 0xd000-0xdfff:    261    415    277      0
+	// 0xe000-0xefff:    516    423    297      0
+	// 0xf000-0xffff:   2785   1903   1773  15450
 }
 
 const data = `