Piano: Allow multiple tracks internally

This commit adds multi-track functionality without exposing it to the
user.

All I really did was rename AudioEngine to Track and allow more than one
Track in TrackManager. A lot of the changes are just changing widgets to
take a TrackManager and use current_track().

The TrackManager creates Tracks and gives them a read-only reference to
the global time value. When the TrackManager wants to fill a sample in
the buffer (in fill_buffer()), it calls fill_sample() on each Track.

The delay code is slightly different - a Track will fill its
m_delay_buffer with the sample it just created rather than the most
recent sample in the buffer (which used to be the same thing).

TrackManager manages the current octave.

Other than those few things, this is a pretty basic separation of
concerns.

Applications/Piano/TrackManager.cpp

@@ -0,0 +1,97 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2019-2020, William McPherson <willmcpherson2@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "TrackManager.h"
+
+TrackManager::TrackManager()
+{
+    add_track();
+}
+
+TrackManager::~TrackManager()
+{
+}
+
+void TrackManager::fill_buffer(FixedArray<Sample>& buffer)
+{
+    memset(buffer.data(), 0, buffer_size);
+
+    for (size_t i = 0; i < buffer.size(); ++i) {
+        for (auto& track : m_tracks)
+            track->fill_sample(buffer[i]);
+
+        if (++m_time >= roll_length) {
+            m_time = 0;
+            if (!m_should_loop)
+                break;
+        }
+    }
+
+    memcpy(m_back_buffer_ptr->data(), buffer.data(), buffer_size);
+    swap(m_front_buffer_ptr, m_back_buffer_ptr);
+}
+
+void TrackManager::reset()
+{
+    memset(m_front_buffer.data(), 0, buffer_size);
+    memset(m_back_buffer.data(), 0, buffer_size);
+
+    m_front_buffer_ptr = &m_front_buffer;
+    m_back_buffer_ptr = &m_back_buffer;
+
+    m_time = 0;
+
+    for (auto& track : m_tracks)
+        track->reset();
+}
+
+void TrackManager::set_note_current_octave(int note, Switch switch_note)
+{
+    current_track().set_note(note + octave_base(), switch_note);
+}
+
+void TrackManager::set_octave(Direction direction)
+{
+    if (direction == Up) {
+        if (m_octave < octave_max)
+            ++m_octave;
+    } else {
+        if (m_octave > octave_min)
+            --m_octave;
+    }
+}
+
+void TrackManager::add_track()
+{
+    m_tracks.append(make<Track>(m_time));
+}
+
+void TrackManager::next_track()
+{
+    if (++m_current_track >= m_tracks.size())
+        m_current_track = 0;
+}