change command buffers to be pooled

src/CommandBuffer.cs

@@ -4,16 +4,16 @@ using RefreshCS;
 
 namespace Campari
 {
-    public struct CommandBuffer
+    public class CommandBuffer
     {
         public GraphicsDevice Device { get; }
-        public IntPtr Handle { get; }
+        public IntPtr Handle { get; internal set; }
 
         // called from RefreshDevice
-        internal CommandBuffer(GraphicsDevice device, IntPtr handle)
+        internal CommandBuffer(GraphicsDevice device)
         {
             Device = device;
-            Handle = handle;
+            Handle = IntPtr.Zero;
         }
 
         public void BeginRenderPass(

src/GraphicsDevice.cs

@@ -1,7 +1,5 @@
 using System;
 using System.Collections.Generic;
-using System.Runtime.InteropServices;
-using System.Text;
 using RefreshCS;
 
 namespace Campari
@@ -12,10 +10,13 @@ namespace Campari
 
         public bool IsDisposed { get; private set; }
 
+        private Queue<CommandBuffer> commandBufferPool;
+
         public GraphicsDevice(
             IntPtr deviceWindowHandle,
             Refresh.PresentMode presentMode,
-            bool debugMode
+            bool debugMode,
+            int initialCommandBufferPoolSize = 4
         ) {
             var presentationParameters = new Refresh.PresentationParameters
             {
@@ -27,13 +28,26 @@ namespace Campari
                 ref presentationParameters,
                 Conversions.BoolToByte(debugMode)
             );
+
+            commandBufferPool = new Queue<CommandBuffer>(initialCommandBufferPoolSize);
+            for (var i = 0; i < initialCommandBufferPoolSize; i += 1)
+            {
+                commandBufferPool.Enqueue(new CommandBuffer(this));
+            }
         }
 
-        /* FIXME: pool this */
         public CommandBuffer AcquireCommandBuffer()
         {
             var commandBufferHandle = Refresh.Refresh_AcquireCommandBuffer(Handle, 0);
-            return new CommandBuffer(this, commandBufferHandle);
+            if (commandBufferPool.Count == 0)
+            {
+                commandBufferPool.Enqueue(new CommandBuffer(this));
+            }
+
+            var commandBuffer = commandBufferPool.Dequeue();
+            commandBuffer.Handle = commandBufferHandle;
+
+            return commandBuffer;
         }
 
         public unsafe void Submit(params CommandBuffer[] commandBuffers)
@@ -50,6 +64,13 @@ namespace Campari
                 (uint) commandBuffers.Length,
                 (IntPtr) commandBufferPtrs
             );
+
+            // return to pool
+            for (var i = 0; i < commandBuffers.Length; i += 1)
+            {
+                commandBuffers[i].Handle = IntPtr.Zero;
+                commandBufferPool.Enqueue(commandBuffers[i]);
+            }
         }
 
         protected virtual void Dispose(bool disposing)