image/jpeg: improve decoder performance by ~12%

[romshark]

improve JPEG decoder performance by about 10-14% on most
architectures by unrolling unzig and shift-clamp loops.
The trade-off is ~16KiB additional binary size.

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[romshark]

The benchmark results and experimentation playground is here
https://github.com/romshark/jpegbench

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[EliCDavis]

A potential improvement if the dst array is 0'd out

	writeDst := func(index int) {
		c := (*b)[index]
		if c < -127 {
			return
		}

		c += 128
		if c > 255 {
			c = 255
		}
		dst[(index/8)*stride+(index%8)] = uint8(c)
	}

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