Read PNG size from raw binary data

png-size.r3

Rebol [
	name:  png-size
	needs: 3.2.0
	usage: [
		png-size http://avatars-04.gitter.im/gh/uv/4/oldes
	]
]

png-size: func[
	"Read PNG size from raw binary data"
	bin [binary! file! url!]
][
	;- there must be at least 24 bytes, but as IHDR does not
	;- have to be the first chunk, it is safer to read more..
	unless binary? bin [ bin: read/binary/part bin 256 ]
	if all [
		bin: find/match bin #{89504E470D0A1A0A} ;- PNG magic number
		bin: find/tail  bin #{49484452}         ;- IHDR chunk
		8 <= length? bin
	][       
		to pair! binary/read bin [UI32 UI32]
	]
]