Improve JPEG processing efficiency by avoiding unnecessary decode (#345) #259

Job	Run time
clippy	20s
pdfutil	20s
build (stable)	34s
build (beta)	41s
build (nightly)	37s
build (feat: no default features)	33s
build (feat: all features)	1m 11s
build (feat: nom only)	36s
	4m 52s

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