FEAT: enable loading exact frame from an image on macOS

Author: Oldes

src/core/n-image.c

@@ -364,7 +364,7 @@ typedef struct REBCLR {
 */	REBNATIVE(image)
 /*
 //  image: native [
-//		"Interface to basic image encoding/decoding (only on Windows so far!)"
+//		"Interface to basic image encoding/decoding (only on Windows and macOS so far!)"
 //		/load      "Image file to load or binary to decode"
 //		 src-file  [file! binary!]
 //		/save      "Encodes image to file or binary"

src/os/osx/sys-codecs.m

@@ -73,7 +73,7 @@ int DecodeImageFromFile(const char *uri, unsigned int frame, REBCDI *codi)
 			binSrc = CFDataCreateWithBytesNoCopy(NULL, codi->data, codi->len, NULL);
 			imgSrc = CGImageSourceCreateWithData(binSrc, 0);
 		}
-		img = CGImageSourceCreateImageAtIndex(imgSrc, 0, 0);
+		img = CGImageSourceCreateImageAtIndex(imgSrc, frame, 0);
 		ASSERT_NOT_NULL(img, 3, "create an image");
 		w = CGImageGetWidth(img);
 		h = CGImageGetHeight(img);

src/tests/units/codecs-test.r3

@@ -549,6 +549,16 @@ if find codecs 'GIF [
 			img/rgb = #{C800000000C800C800FFFFFF}
 		]
 		try [delete %new.gif]
+
+	--test-- "animated gif"
+		--assert try [all [
+			i1: image/load/frame %units/files/animation.gif 1
+			i2: image/load/frame %units/files/animation.gif 2
+			i3: image/load/frame %units/files/animation.gif 3
+			#{4D99990699791F57F238C7195ABB0DE7} == checksum i1/rgba 'md5
+			#{878BACEFB949C6435702F87D5B62F9FA} == checksum i2/rgba 'md5
+			#{1414A649C4CF2E2D9DE3A8502A4425AA} == checksum i3/rgba 'md5
+		]]
 	===end-group===
 ]