Added support for creating image from a sub image.

- With a non-zero Min coordinate.

allg5/bitmap.go

@@ -103,10 +103,11 @@ func NewBitmapFromImage(src image.Image, video bool) (*Bitmap, error) {
 		return nil, errors.New("unable to lock bitmap")
 	}
 	dst := (*[1 << 30]uint8)(region.data)
+	left, top := bnd.Min.X, bnd.Min.Y
 	for y := 0; y < height; y++ {
 		row := dst[y*int(region.pitch):]
 		for x := 0; x < width; x++ {
-			r, g, b, a := src.At(x, y).RGBA()
+			r, g, b, a := src.At(left+x, top+y).RGBA()
 			row[x*4] = uint8(r >> 8)
 			row[x*4+1] = uint8(g >> 8)
 			row[x*4+2] = uint8(b >> 8)