Potential workaround for poor zstd performance

Performance is still not great, but better

reader_frag.go

@@ -0,0 +1,22 @@
+package squashfs
+
+import (
+	"io"
+
+	"github.com/CalebQ42/squashfs/internal/toreader"
+)
+
+type fragEntry struct {
+	Start uint64
+	Size  uint32
+	_     uint32
+}
+
+func (r Reader) fragReader(index uint32) (io.Reader, error) {
+	realSize := r.fragEntries[index].Size &^ (1 << 24)
+	rdr := io.LimitReader(toreader.NewReader(r.r, int64(r.fragEntries[index].Start)), int64(realSize))
+	if realSize != r.fragEntries[index].Size {
+		return rdr, nil
+	}
+	return r.d.Reader(rdr)
+}