Fix HDRP VT test compilation. (#650)

* Changed use of VT.System namespace to VT.Streaming namespace where the API has changed.

* Updated min Unity versions of SRP packages to 2020.2.0a14

* Fixed compilation of HDRP VT test

Co-authored-by: Nico Leyman <[email protected]>

Author: NicoLeyman

TestProjects/HDRP_RuntimeTests/Assets/Scenes/003-VirtualTexturing/LowResolutionRequester.cs

@@ -45,7 +45,7 @@ void Update()
         {
             foreach (int prop in properties)
             {
-                UnityEngine.Rendering.VirtualTexturing.System.RequestRegion(materialToRequest, prop, new Rect(0.0f, 0.0f, 1.0f, 1.0f), firstMipToRequest, UnityEngine.Rendering.VirtualTexturing.System.AllMips);
+                UnityEngine.Rendering.VirtualTexturing.Streaming.RequestRegion(materialToRequest, prop, new Rect(0.0f, 0.0f, 1.0f, 1.0f), firstMipToRequest, UnityEngine.Rendering.VirtualTexturing.System.AllMips);
             }
         }
 #endif