Combined duplicate channels in sound engine

[jroweboy]

If two channels are unused, then they very likely have the exact same data. This change will detect when a channel's data is already in the file, and point the header pointer to the previous channel data to reduce size for no extra cost

Example:

Consider the following header

music_data_oddity:
	.byte 7
	.word @instruments
	.word @instruments_exp
	.word @samples-4
	.word @song0ch0,@song0ch1,@song0ch2 ; 00 : EPSM Detected

@song0ch0:
  .byte $ff, $ff
@song0ch1:
  .byte $ff, $ff
@song0ch2:
  .byte $ff, $ff

With this patch, the export code will detect that the data is exactly the same between the channels and merge them like so

music_data_oddity:
	.byte 7
	.word @instruments
	.word @instruments_exp
	.word @samples-4
	.word @song0ch0,@song0ch1,@song0ch2 ; 00 : EPSM Detected

@song0ch2:
@song0ch1:
@song0ch0:
  .byte $ff, $ff

The header remains the same, and the data is still there for one channel, but all the "unused" channels will be merged into one.

[BleuBleu]

This one well put on hold until 4.3.0. Too risky.