Allow short palettes support

reader.go

@@ -229,8 +229,8 @@ func (d *decoder) decodeSmallPaletted() (image.Image, error) {
 		return paletted, nil
 	}
 	// There are specified bpp bits per pixel, and each row is 4-byte aligned.
-	cap := 8 / int(d.bpp)
-	b := make([]byte, ((d.c.Width+cap-1)/cap+3)&^3)
+	pixelsPerByte := 8 / int(d.bpp)
+	b := make([]byte, ((d.c.Width+pixelsPerByte-1)/pixelsPerByte+3)&^3)
 	y0, y1, yDelta := d.c.Height-1, -1, -1
 	if d.topDown {
 		y0, y1, yDelta = 0, d.c.Height, +1

writer.go

@@ -247,15 +247,19 @@ func Encode(w io.Writer, m image.Image) error {
 			h.bpp = 4
 		default:
 			h.bpp = 8
-			step = (d.X + 3) &^ 3
+		}
+		colors := 1 << h.bpp
+		if len(m.Palette) < 1<<h.bpp {
+			colors = len(m.Palette)
+			h.colorUse = uint32(colors)
 		}
 		if h.bpp < 8 {
-			cap := 8 / int(h.bpp)
-			step = ((d.X+cap-1)/cap + 3) &^ 3
+			pixelsPerByte := 8 / int(h.bpp)
+			step = ((d.X+pixelsPerByte-1)/pixelsPerByte + 3) &^ 3
 		} else {
 			step = (d.X + 3) &^ 3
 		}
-		palette = make([]byte, 1<<h.bpp*4)
+		palette = make([]byte, colors*4)
 		for i := 0; i < len(m.Palette) && i < 1<<h.bpp; i++ {
 			r, g, b, _ := m.Palette[i].RGBA()
 			palette[i*4+0] = uint8(b >> 8)