FEAT: extended PNG codec with chunks function

It's useful for close examination of PNG file data chunks and also for removing or adding some chunks manually.

For example this short code rewrites PNG keeping only critical PNG chunks:
```rebol
chunks: :codecs/png/chunks
write %reduced.png chunks chunks/only %source.png ["IDAT" "IHDR" "PLTE"]
```

src/mezz/codec-image-ext.reb

@@ -2,10 +2,11 @@ REBOL [
 	title:  "REBOL 3 image codecs extensions"
 	name:   'codec-image-ext
 	author: "Oldes"
-	version: 0.1.0
-	date:    10-Nov-2020
+	version: 0.2.0
+	date:    8-Mar-2021
 	history: [
 		0.1.0 10-Nov-2020 "Oldes" {Extend native PNG codec with `size?` function}
+		0.2.0 08-Mar-2021 "Oldes" {Extend native PNG with `chunks` function}
 	]
 ]
 
@@ -22,20 +23,92 @@ if find codecs 'png [
 			to pair! binary/read bin [UI32 UI32]
 		]
 	]
-]
-
-
-
-
-
-
-
-
-
-
-
-
-
-
+
+	extend codecs/png 'chunks function [
+		"Decode PNG into block of chunks (or encode back to binary from a block)"
+		data [binary! file! url! block!] "Input data"
+		/only tags [block!]
+	][
+		if block? data [
+			;- Composing previously decoded chunks back to binary...
+			if #{49484452} <> data/1 [
+				sys/log/error 'PNG ["First chunk must be IHDR, but is:" as-red mold to string! tag]
+				return none
+			]
+			out: binary 10000
+			binary/write out #{89504E470D0A1A0A}
+			foreach [tag dat] data [
+				if tag = #{49454E44} [continue] ; IEND is added automatically
+				unless all [binary? tag 4 = length? tag binary? dat][
+					sys/log/error 'PNG ["Wrong chunk input!" as-red tag]
+					return none
+				]
+				len: length? dat
+				binary/write out [ui32be :len]
+				tmp: out/buffer-write
+				binary/write out [:tag :dat]
+				crc: checksum tmp 'crc32
+				binary/write out [ui32be :crc]
+			]
+			; add IEND
+			binary/write out #{0000000049454E44AE426082}
+			return copy out/buffer
+		]
 
+		;- Examine inner content of PNG files converting it to block of chunks...
+		if only [
+			; make sure that all tag ids are as binary
+			forall tags [
+				unless binary? tags/1 [change tags to binary! form tags/1]
+			]
+		]
 
+		unless binary? data [ data: read data ]
+		sys/log/info 'PNG ["^[[1;32mDecode PNG data^[[m (^[[1m" length? data "^[[mbytes )"]
+		unless parse data [#{89504E470D0A1A0A} data: to end][ return none ]
+		bin: binary data
+		out: make block! 12
+		num: -40 + query/mode console:// 'window-cols
+		while [8 < length? bin/buffer][
+			len: binary/read bin 'ui32be
+			tag: copy/part bin/buffer 4
+			; check if we are interested in specific tags
+			if all [tags none? find tags tag][
+				; ignore this tag
+				sys/log/more 'PNG rejoin [form tag #" " as-red to string! tag " ^[[33m" pad len 10 "^[[35mignored"]
+				bin/buffer: skip bin/buffer len + 8
+				continue
+			]
+			; count checksum of this chunk...
+			crc: checksum/part bin/buffer 'crc32 len + 4
+			; skip tag which we already know...
+			bin/buffer: skip bin/buffer 4
+			; get raw data...
+			dat: binary/read/with bin 'BYTES len
+			; read CRC and compare with computed value...
+			if crc <> binary/read bin 'si32be [
+				sys/log/error 'PNG "CRC check failed!"
+				return none
+			]
+			; use some user friendly info output for specific chunks...
+			info: switch/default tag [
+				#{49484452} ;IHDR
+							[binary/read dat [UI32 UI32 UI8 UI8 UI8 UI8 UI8]]
+				#{70485973} ;pHYs
+							[binary/read dat [UI32 UI32 UI8]]
+				#{69545874} ;iTXt
+				#{74455874} ;tEXt
+							[to string! dat]
+			][	dat	]
+			info: mold/flat/part info num + 3
+			; cropped to fit on single line...
+			if num < length? info [ change skip tail info -3 "..." ]
+			; output info...
+			sys/log/more 'PNG rejoin [form tag #" " as-red to string! tag " ^[[33m" pad len 10 info]
+			; store data...
+			append/only append out tag dat
+		]
+		new-line/skip out true 2
+		out
+	]
+]