Re-added NewReaderAtOffset

internal/toreader/offsetreader.go

@@ -0,0 +1,19 @@
+package toreader
+
+import "io"
+
+type OffsetReader struct {
+	r   io.ReaderAt
+	off int64
+}
+
+func NewOffsetReader(r io.ReaderAt, off int64) *OffsetReader {
+	return &OffsetReader{
+		r:   r,
+		off: off,
+	}
+}
+
+func (r OffsetReader) ReadAt(p []byte, off int64) (n int, e error) {
+	return r.r.ReadAt(p, off+r.off)
+}