Remember bound framebuffer to avoid glGetInteger use. (#1273)

glGetInteger seems to sync with GPU which is less than ideal, and slowing down texture copies.

Ryujinx.Graphics.OpenGL/Framebuffer.cs

@@ -20,9 +20,10 @@ namespace Ryujinx.Graphics.OpenGL
             _colors = new TextureView[8];
         }
 
-        public void Bind()
+        public int Bind()
         {
             GL.BindFramebuffer(FramebufferTarget.Framebuffer, Handle);
+            return Handle;
         }
 
         public void AttachColor(int index, TextureView color)