Work on AudioServer

The center of this is now an ABuffer class in LibAudio.
ABuffer contains ASample, which has two channels (left/right) in
floating point for mixing purposes, in 44100hz.

This means that the loaders (AWavLoader in this case) needs to do some
manipulation to get things in the right format, but that we don't need
to care after format loading is done.

While we're at it, do some correctness fixes. PCM data is unsigned if
it's 8 bit, but 16 bit is signed. And /dev/audio also wants signed 16
bit audio, so give it what it wants.

On top of this, AudioServer now accepts requests to play a buffer.
The IPC mechanism here is pretty much a 1:1 copy-paste from
LibGUI/WindowServer. It can be generalized more in the future, but for
now I want to get AudioServer working decently first :)

Additionally, add a little "aplay" tool to load and play a WAV file. It
will break with large WAVs (run out of memory, heh...) but it's a start.

Future work needs to make AudioServer block buffer submission from
clients until it has played the buffer they are requesting to play.

Servers/AudioServer/ASMixer.cpp

@@ -0,0 +1,112 @@
+#include <AK/BufferStream.h>
+#include <LibCore/CThread.h>
+
+#include <limits>
+#include "ASMixer.h"
+
+ASMixer::ASMixer()
+    : m_device("/dev/audio")
+{
+    if (!m_device.open(CIODevice::WriteOnly)) {
+        dbgprintf("Can't open audio device: %s\n", m_device.error_string());
+        return;
+    }
+
+    CThread sound_thread([](void* context) -> int {
+        ASMixer* mixer = (ASMixer*)context;
+        mixer->mix();
+        return 0;
+    }, this);
+}
+
+void ASMixer::queue(ASClientConnection&, const ABuffer& buffer)
+{
+    ASSERT(buffer.size_in_bytes());
+    CLocker lock(m_lock);
+    m_pending_mixing.append(ASMixerBuffer(buffer));
+}
+
+void ASMixer::mix()
+{
+    Vector<ASMixerBuffer> active_mix_buffers;
+
+    for (;;) {
+        {
+            CLocker lock(m_lock);
+            for (const auto& buf : m_pending_mixing)
+                active_mix_buffers.append(buf);
+            m_pending_mixing.clear();
+        }
+
+        // ### use a wakeup of some kind rather than this garbage
+        if (active_mix_buffers.size() == 0) {
+            // nothing to mix yet
+            usleep(10000);
+            continue;
+        }
+
+        int max_size = 0;
+
+        for (auto& buffer : active_mix_buffers) {
+            if (buffer.done)
+                continue;
+            ASSERT(buffer.buffer->size_in_bytes()); // zero sized buffer? how?
+            max_size = max(max_size, buffer.buffer->size_in_bytes() - buffer.pos);
+        }
+
+        // ### clear up 'done' buffers more aggressively
+        if (max_size == 0) {
+            active_mix_buffers.clear();
+            continue;
+        }
+
+        max_size = min(1023, max_size);
+
+        Vector<ASample> mixed_buffer;
+        mixed_buffer.resize(max_size);
+
+        // Mix the buffers together into the output
+        for (auto& buffer : active_mix_buffers) {
+            if (buffer.done)
+                continue;
+            auto& samples = buffer.buffer->samples();
+
+            for (int i = 0; i < max_size && buffer.pos < samples.size(); ++buffer.pos, ++i) {
+                auto& mixed_sample = mixed_buffer[i];
+                mixed_sample += samples[buffer.pos];
+            }
+
+            // clear it later
+            if (buffer.pos == samples.size())
+                buffer.done = true;
+        }
+
+        // output the mixed stuff to the device
+        // max_size is 0 indexed, so add 1.
+        const int output_buffer_byte_size = (max_size + 1) * 2 * 2;
+        ASSERT(output_buffer_byte_size == 4096);
+        ByteBuffer buffer(ByteBuffer::create_uninitialized(output_buffer_byte_size));
+        BufferStream stream(buffer);
+
+        for (int i = 0; i < mixed_buffer.size(); ++i) {
+            auto& mixed_sample = mixed_buffer[i];
+            mixed_sample.clamp();
+
+            i16 out_sample;
+            out_sample = mixed_sample.left * std::numeric_limits<i16>::max();
+            stream << out_sample;
+
+            ASSERT(!stream.at_end()); // we should have enough space for both channels in one buffer!
+            out_sample = mixed_sample.right * std::numeric_limits<i16>::max();
+            stream << out_sample;
+
+            ASSERT(!stream.at_end());
+        }
+
+        if (stream.offset() != 0) {
+            buffer.trim(stream.offset());
+            m_device.write(buffer);
+            mixed_buffer.resize(0);
+        }
+    }
+}