Incorrect buffer size calculated

[adamrenklint]

• Consider a sample that is a perfect 1-bar loop (2 sec) at 120 bpm
• We create a pattern with 60 bpm
• We pitch the sample -12 semitones
• We'd expect the pitched-down sample to be a perfect fit
• But instead the sample will be too short, cut off too early

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BufferHelper.getCopyLength does not take the playbackRate of a sample into consideration: https://github.com/adamrenklint/bap/blob/master/lib/BufferHelper.js#L73