resources/images: Fix 2 animated GIF resize issues

* Fix resize of animated GIF when target != GIF
* Avoid processing all GIF frames if targetFormat != GIF

Fixes #10354

resources/image_test.go

@@ -620,6 +620,8 @@ func TestImageOperationsGolden(t *testing.T) {
 
 	// Note, if you're enabling this on a MacOS M1 (ARM) you need to run the test with GOARCH=amd64.
 	// GOARCH=amd64 go test -timeout 30s -run "^TestImageOperationsGolden$" ./resources -v
+	// The above will print out a folder.
+	// Replace testdata/golden with resources/_gen/images in that folder.
 	devMode := false
 
 	testImages := []string{"sunset.jpg", "gohugoio8.png", "gohugoio24.png"}
@@ -663,7 +665,7 @@ func TestImageOperationsGolden(t *testing.T) {
 
 	// Animated GIF
 	orig = fetchImageForSpec(spec, c, "giphy.gif")
-	for _, resizeSpec := range []string{"200x", "512x"} {
+	for _, resizeSpec := range []string{"200x", "512x", "100x jpg"} {
 		resized, err := orig.Resize(resizeSpec)
 		c.Assert(err, qt.IsNil)
 		rel := resized.RelPermalink()

resources/images/config.go

@@ -60,6 +60,7 @@ var (
 	imageFormatsVersions = map[Format]int{
 		PNG:  3, // Fix transparency issue with 32 bit images.
 		WEBP: 2, // Fix transparency issue with 32 bit images.
+		GIF:  1, // Fix resize issue with animated GIFs when target != GIF.
 	}
 
 	// Increment to mark all processed images as stale. Only use when absolutely needed.

resources/images/image.go

@@ -247,7 +247,7 @@ func (p *ImageProcessor) ApplyFiltersFromConfig(src image.Image, conf ImageConfi
 		return nil, fmt.Errorf("unsupported action: %q", conf.Action)
 	}
 
-	img, err := p.Filter(src, filters...)
+	img, err := p.doFilter(src, conf.TargetFormat, filters...)
 	if err != nil {
 		return nil, err
 	}
@@ -256,11 +256,18 @@ func (p *ImageProcessor) ApplyFiltersFromConfig(src image.Image, conf ImageConfi
 }
 
 func (p *ImageProcessor) Filter(src image.Image, filters ...gift.Filter) (image.Image, error) {
+	return p.doFilter(src, 0, filters...)
+}
+
+func (p *ImageProcessor) doFilter(src image.Image, targetFormat Format, filters ...gift.Filter) (image.Image, error) {
 
 	filter := gift.New(filters...)
 
-	if giph, ok := src.(Giphy); ok && len(giph.GIF().Image) > 1 {
+	if giph, ok := src.(Giphy); ok {
 		g := giph.GIF()
+		if len(g.Image) < 2 || (targetFormat == 0 || targetFormat != GIF) {
+			src = g.Image[0]
+		} else {
 			var bounds image.Rectangle
 			firstFrame := g.Image[0]
 			tmp := image.NewNRGBA(firstFrame.Bounds())
@@ -277,6 +284,8 @@ func (p *ImageProcessor) Filter(src image.Image, filters ...gift.Filter) (image.
 			return giph, nil
 		}
 
+	}
+
 	bounds := filter.Bounds(src.Bounds())
 
 	var dst draw.Image