lass: only allocate amp buffer if used by clipping mode

[passyur]

cuts like ~30% of memory allocs

basically, each Track has two SoundSample buffers—amp and wave—of identical size (typically on the order of MB). Was often the case that amp was not used by the clipping mode but was still being allocated. Claude says this:

Per-channel unit — 8-channel piece, 1 minute, 44.1 kHz:

samples/channel = 60 × 44100 = 2,646,000
wave buffer/channel = 2,646,000 × 4 B ≈ 10.1 MB
old Track/channel = wave + amp ≈ 20.2 MB (half of it amp)
final 8-channel score buffer: old ≈ 162 MB (81 wave + 81 amp) → new ≈ 81 MB