Simple GPU fixes (#1093)

* Implement RasterizeEnable

* Match viewport count to hardware

* Simplify ScissorTest tracking around Blits

* Disable RasterizerDiscard around Blits and track its state

* Read RasterizeEnable reg as bool and add doc

Ryujinx.Graphics.Gpu/Constants.cs

@@ -53,6 +53,6 @@ namespace Ryujinx.Graphics.Gpu
         /// <summary>
         /// Maximum number of viewports.
         /// </summary>
-        public const int TotalViewports = 8;
+        public const int TotalViewports = 16;
     }
 }