Load operations (#105)

* load ops: design + implementation (all backends)

* Animometer/glTFViewer: use just one subpass per frame

src/backend/Texture.cpp

@@ -52,6 +52,17 @@ namespace backend {
         }
     }
 
+    bool TextureFormatHasDepthOrStencil(nxt::TextureFormat format) {
+        switch (format) {
+            case nxt::TextureFormat::R8G8B8A8Unorm:
+                return false;
+            case nxt::TextureFormat::D32FloatS8Uint:
+                return true;
+            default:
+                UNREACHABLE();
+        }
+    }
+
 
     // TextureBase