AudioServer: Add a "main mix volume" and a simple program to get/set it

Give the mixer a main volume value (percent) that we scale all the
outgoing samples by (before clipping.)

Also add a simple "avol" program for querying and setting the volume:

- "avol" prints the current volume.
- "avol 200" sets the main mix volume to 200%

Servers/AudioServer/ASClientConnection.cpp

@@ -60,6 +60,20 @@ bool ASClientConnection::handle_message(const ASAPI_ClientMessage& message, cons
         post_message(reply);
         break;
     }
+    case ASAPI_ClientMessage::Type::GetMainMixVolume: {
+        ASAPI_ServerMessage reply;
+        reply.type = ASAPI_ServerMessage::Type::DidGetMainMixVolume;
+        reply.value = m_mixer.main_volume();
+        post_message(reply);
+        break;
+    }
+    case ASAPI_ClientMessage::Type::SetMainMixVolume: {
+        ASAPI_ServerMessage reply;
+        reply.type = ASAPI_ServerMessage::Type::DidSetMainMixVolume;
+        m_mixer.set_main_volume(message.value);
+        post_message(reply);
+        break;
+    }
     case ASAPI_ClientMessage::Type::Invalid:
     default:
         dbgprintf("ASClientConnection: Unexpected message ID %d\n", int(message.type));

Servers/AudioServer/ASMixer.cpp

@@ -71,6 +71,8 @@ void ASMixer::mix()
 
         for (int i = 0; i < mixed_buffer_length; ++i) {
             auto& mixed_sample = mixed_buffer[i];
+
+            mixed_sample.scale(m_main_volume);
             mixed_sample.clip();
 
             i16 out_sample;

Servers/AudioServer/ASMixer.h

@@ -54,11 +54,16 @@ public:
 
     NonnullRefPtr<ASBufferQueue> create_queue(ASClientConnection&);
 
+    int main_volume() const { return m_main_volume; }
+    void set_main_volume(int volume) { m_main_volume = volume; }
+
 private:
     Vector<NonnullRefPtr<ASBufferQueue>> m_pending_mixing;
 
     CFile m_device;
     CLock m_lock;
 
+    int m_main_volume { 100 };
+
     void mix();
 };