Fixed bug with negative pitch.

bitmap.go

@@ -231,12 +231,23 @@ func (b *Bitmap) Image() image.Image {
 		return nil
 	}
 	defer C.al_unlock_bitmap(b.bitmap)
-	src := (*[1 << 30]uint8)(region.data)
+
-	dst := im.Pix
+	pitch := int(region.pitch)
+	length := b.height * pitch
+	data := uintptr(region.data)
 	var srcOff, dstOff int
+	if pitch < 0 {
+		srcOff = (b.height - 1) * -pitch
+		length *= -1
+		data = data - uintptr(srcOff)
+	}
+
+	rowLength := b.width * 4
+	src := (*[1 << 28]uint8)(unsafe.Pointer(data))[:length:length]
+	dst := im.Pix
 	for y := 0; y < b.height; y++ {
-		copy(dst[dstOff:], src[srcOff:srcOff+b.width*4])
+		copy(dst[dstOff:], src[srcOff:srcOff+rowLength])
-		srcOff += int(region.pitch)
+		srcOff += pitch
 		dstOff += im.Stride
 	}
 	return im