graphics: Bug fix: Wrong palette usage when the palette is big

internal/graphics/image.go

@@ -51,7 +51,7 @@ func CopyImage(origImg image.Image) *image.RGBA {
 		pix1 := newImg.Pix
 		for j := 0; j < y1-y0; j++ {
 			for i := 0; i < x1-x0; i++ {
-				p := pix0[index0]
+				p := int(pix0[index0])
 				pix1[index1] = palette[4*p]
 				pix1[index1+1] = palette[4*p+1]
 				pix1[index1+2] = palette[4*p+2]

internal/graphics/image_test.go

@@ -28,6 +28,15 @@ func TestCopyImage(t *testing.T) {
 	for i := range pal {
 		pal[i] = color.White
 	}
+	p := make([]color.Color, 255)
+	for i := range p {
+		if i == 64 {
+			p[i] = color.White
+		} else {
+			p[i] = color.Transparent
+		}
+	}
+	bigPalette := color.Palette(p)
 	cases := []struct {
 		In  image.Image
 		Out *image.RGBA
@@ -67,6 +76,19 @@ func TestCopyImage(t *testing.T) {
 				Rect:   image.Rect(0, 0, 2, 2),
 			},
 		},
+		{
+			In: &image.Paletted{
+				Pix:     []uint8{0, 64, 0, 0},
+				Stride:  2,
+				Rect:    image.Rect(0, 0, 2, 2),
+				Palette: bigPalette,
+			},
+			Out: &image.RGBA{
+				Pix:    []uint8{0, 0, 0, 0, 0xff, 0xff, 0xff, 0xff, 0, 0, 0, 0, 0, 0, 0, 0},
+				Stride: 8,
+				Rect:   image.Rect(0, 0, 2, 2),
+			},
+		},
 	}
 	for _, c := range cases {
 		got := CopyImage(c.In)