Add support for decoding an animated WebP

The initial implementation doesn't fully expose `WebPAnimDecoderOptions`.

webp/anim_decode_test.go

@@ -0,0 +1,77 @@
+package webp_test
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/harukasan/go-libwebp/test/util"
+	"github.com/harukasan/go-libwebp/webp"
+)
+
+func TestDecodeAnimationInfo(t *testing.T) {
+	data := util.ReadFile("weather-anim.webp")
+
+	dec, err := webp.NewAnimationDecoder(data)
+	if err != nil {
+		t.Fatalf("initializing decoder: %v", err)
+	}
+	defer dec.Close()
+
+	info, err := dec.GetInfo()
+	if err != nil {
+		t.Fatalf("getting animatiion info: %v", err)
+	}
+
+	if got := info.CanvasWidth; got != 64 {
+		t.Errorf("Expected CanvasWidth: %v, but got %v", 64, got)
+	}
+	if got := info.CanvasHeight; got != 32 {
+		t.Errorf("Expected CanvasHeight: %v, but got %v", 32, got)
+	}
+	if got := info.LoopCount; got != 0 {
+		t.Errorf("Expected LoopCount: %v, but got %v", 0, got)
+	}
+	if got := info.FrameCount; got != 18 {
+		t.Errorf("Expected FrameCount: %v, but got %v", 18, got)
+	}
+}
+
+func TestDecodeAnimation(t *testing.T) {
+	data := util.ReadFile("weather-anim.webp")
+
+	dec, err := webp.NewAnimationDecoder(data)
+	if err != nil {
+		t.Fatalf("initializing decoder: %v", err)
+	}
+	defer dec.Close()
+
+	anim, err := dec.Decode()
+	if err != nil {
+		t.Fatalf("error decoding: %v", err)
+	}
+
+	if got := len(anim.Image); got != 18 {
+		t.Errorf("Expected len(Image): %v, but got %v", 18, got)
+	}
+	if got := len(anim.Timestamp); got != 18 {
+		t.Errorf("Expected len(Timestamp): %v, but got %v", 18, got)
+	}
+
+	for i := 0; i < 18; i++ {
+		frame := util.ReadPNG(fmt.Sprintf("weather-anim-frames/%02d.png", i))
+		got := anim.Image[i]
+
+		for y := 0; y < got.Bounds().Dy(); y++ {
+			for x := 0; x < got.Bounds().Dx(); x++ {
+				if got.At(x, y) != frame.At(x, y) {
+					t.Fatalf(
+						"Expected frame %d (%d,%d): %v, but got %v",
+						i, x, y,
+						frame.At(x, y),
+						got.At(x, y),
+					)
+				}
+			}
+		}
+	}
+}