`E1` panning during held note doesn't emulate correctly

[ChrisBlueStone]

The command [30 7B] A1 [E1 00] C8 A1 holds down the note A1 for 0x60 ticks due to the C8 command, but the panning changes after 0x30 ticks due to the E1 command.

Current behavior

In ebmused, the pan changes after 0x30 ticks in the middle of the note.

Expected behavior

In game, the E1 command has no effect until the second A1 note command is played after 0x60 ticks. Note, that this behavior is different if an E2 command is sliding the pan value when the E1 command is hit. See #33