FEAT: codecs/gif/size? function for resolving gif image size without need of decoding

Author: Oldes

src/mezz/codec-image-ext.reb

@@ -146,3 +146,16 @@ if find codecs 'jpeg [
 		none
 	]
 ]
+
+
+if find codecs 'gif [
+	extend codecs/gif 'size? function ["Return GIF image size or none" img [file! url! binary!]][
+		unless binary? img [img: read/binary/part img 16]
+		parse img [
+			["GIF87a" | "GIF89a"] img: (
+				return to pair! binary/read img [UI16LE UI16LE]
+			) 
+		]
+		none
+	]
+]

src/tests/units/codecs-test.r3

@@ -412,7 +412,15 @@ if find codecs 'JPEG [
 		--assert 256x256 = codecs/jpeg/size? %units/files/flower-tiny.jpg
 		--assert none?     codecs/jpeg/size? %units/files/test.aar
 	===end-group===
-]		
+]
+
+if find codecs 'GIF [
+	===start-group=== "GIF codec"
+	--test-- "gif/size?"
+		--assert 256x256 = codecs/gif/size? %units/files/flower.gif
+		--assert none?     codecs/gif/size? %units/files/test.aar
+	===end-group===
+]	
 
 if find codecs 'XML [
 	===start-group=== "XML codec"