Fix wrong waveform and random crackles in N163 audio

[nyanpasu64]

This pull request aims to fix N163 audio bugs I've discovered.

Changes in this PR:

• Properly initialize Namco163Audio emulation core in constructor.
  • Fixes bug where if a N163 channel uses the Zxx effect to set wave position, the first note plays with corrupted sound until it switches waves or plays a new note.
• Fix random crackles in N163 playback and WAV export.
  • This was caused by N163 Blip_Buffer frames desynchronizing from global frames after reset.

Background

N163 playback, like other chips, uses Blip_Synth and Blip_Buffer for resampling. Each sound chip has its own Blip_Synth, with an independent volume control and treble filter (a set of impulse responses).

Chip emulators create audio by calling Blip_Synth::update(time_clocks, amplitude). Blip_Synth calculates delta = amplitude - previous amplitude, then inserts an impulse of size delta into a Blip_Buffer. Blip_Synth performs resampling by converting time_clocks into fractional samples, then adding a fractionally delayed impulse to Blip_Buffer's difference array starting at int(sample timestamp). (This introduces a few samples of delay, from the impulse start to center point.)

Each frame lasts an integer number of clocks, but a fractional number of samples. At the end of a frame, Blip_Buffer::read_samples() finds the fractional sample where the next frame begins, generates audio up to int(sample timestamp) using a running sum of Blip_Buffer's difference array (with optional highpass DC removal), then outputs and discards int(sample timestamp) samples (keeping the fractional part of sample timestamp, so the next frame's impulses begin midway through the new first sample of the buffer).

Bug

Dn-FT outputs audio from a global Blip_Buffer, CMixer::BlipBuffer. In order to apply (approximately emulated) audio filtering, CN163 has its own Blip_Buffer, CN163::m_BlipN163.

When we begin playback, we call CAPU::Reset() → CMixer::ClearBuffer() and CN163::Reset(), which both reset Blip_Buffer::offset_ to 0. When loading a document with a nonstandard tick rate or pressing F12, CSoundGen and CAPU's frames can desync, causing CAPU to get reset outside of CAPU::EndFrame() boundaries.

The bug in this case was that when FamiTracker called CN163::Reset() without ending the previous frame normally, CN163 failed to clear its "time since frame begin" field (CN163::m_iTime). Normally this would result in steps being delayed slightly for the next frame. But CN163 has its own Blip_Buffer, so each subsequent frame starts and stops at a different fractional sample than the global Blip_Buffer, resulting in different numbers of completed samples being generated each frame.

These frames of incorrect length get mixed into the global Blip_Buffer when CN163::EndFrame() calls Output.mix_samples_raw() (where Output points to CMixer::BlipBuffer). Whenever a N163 frame doesn't match the length of its corresponding global frame, it can produce 1-sample overlaps or gaps at frame borders, resulting in audio crackling.

Fix

Make CN163::Reset() set m_iTime = 0, so the first N163 frame adds samples at the correct time, has the correct length, and all subsequent frames have the correct phase relative to the global Blip_Buffer.

Does CN163 need to have a separate Blip_Buffer? It's complicated. It's used to perform a first-order lowpass filter at 12 kHz, but this is close enough to Nyquist (at the usual sampling rate of 44-48 kHz) to have noticeable frequency warping. I don't know if this is hardware-accurate or an attempt to filter out switching noise.