audio/internal/convert: Seek with incomplete bytes should work

Usually users use audio.Player objects and doesn't use Resampling
object directly. Resampling object itself is exposed by
audio.Resample, but this is usually used with audio.Player objects.
Thus, this issue is not so serious in the real world.

audio/internal/convert/resampling.go

@@ -298,5 +298,7 @@ func (r *Resampling) Seek(offset int64, whence int) (int64, error) {
 	if r.Length() <= r.pos {
 		r.pos = r.Length()
 	}
+	size := r.bytesPerSample()
+	r.pos = r.pos / int64(size) * int64(size)
 	return r.pos, nil
 }

audio/internal/convert/resampling_test.go

@@ -89,10 +89,32 @@ func TestResampling(t *testing.T) {
 				t.Run(fmt.Sprintf("bitDepthInBytes=%d", bitDepthInBytes), func(t *testing.T) {
 					inB := newSoundBytes(c.In, bitDepthInBytes)
 					outS := convert.NewResampling(bytes.NewReader(inB), int64(len(inB)), c.In, c.Out, bitDepthInBytes)
-					gotB, err := io.ReadAll(outS)
+					var gotB []byte
+					for {
+						var buf [97]byte
+						n, err := outS.Read(buf[:])
+						gotB = append(gotB, buf[:n]...)
+						if err != nil {
+							if err != io.EOF {
+								t.Fatal(err)
+							}
+							break
+						}
+						cur, err := outS.Seek(0, io.SeekCurrent)
 						if err != nil {
 							t.Fatal(err)
 						}
+						// Shifting by incomplete bytes should not affect the result.
+						for i := 0; i < bitDepthInBytes*2-1; i++ {
+							pos, err := outS.Seek(int64(i), io.SeekCurrent)
+							if err != nil {
+								t.Fatal(err)
+							}
+							if cur != pos {
+								t.Errorf("cur: %d, pos: %d", cur, pos)
+							}
+						}
+					}
 					wantB := newSoundBytes(c.Out, bitDepthInBytes)
 					if len(gotB) != len(wantB) {
 						t.Errorf("len(gotB) == %d but len(wantB) == %d", len(gotB), len(wantB))