Flush buffers and texture data through a persistent mapped buffer. (#2481)

* Use persistent buffers to flush texture data

* Flush buffers via copy to persistent buffers.

* Log error when timing out, small refactoring.

Ryujinx.Graphics.Gpu/Image/Texture.cs

@@ -876,9 +876,9 @@ namespace Ryujinx.Graphics.Gpu.Image
         /// This is not cheap, avoid doing that unless strictly needed.
         /// </remarks>
         /// <returns>Host texture data</returns>
-        private Span<byte> GetTextureDataFromGpu(bool blacklist, ITexture texture = null)
+        private ReadOnlySpan<byte> GetTextureDataFromGpu(bool blacklist, ITexture texture = null)
         {
-            Span<byte> data;
+            ReadOnlySpan<byte> data;
 
             if (texture != null)
             {