Fix a couple minor TBT bugs

- Quiet a false-positive warning.
  alphaMode=null should be treated as Opaque (according to the spec).
  Don't issue a "not yet supported" warning about that case.
- Fix detection of Google Blocks.

Change-Id: Id000b015d6b9800ebc731b352639d3252fb385c5

UnitySDK/Assets/TiltBrush/Scripts/Gltf/GltfMaterialConverter.cs

@@ -256,6 +256,8 @@ private static Material LookUpGlobalMaterial(GltfMaterialBase gltfMaterial) {
       string alphaMode = gltfMat.alphaMode == null ? null : gltfMat.alphaMode.ToUpperInvariant();
 
       switch (alphaMode) {
+      case null:
+      case "":
       case Gltf2Material.kAlphaModeOpaque:
         baseMaterial = gltfMat.doubleSided
             ? TbtSettings.Instance.m_BasePbrOpaqueDoubleSidedMaterial

UnitySDK/Assets/TiltBrush/Scripts/Gltf/ImportGltf.cs

@@ -351,6 +351,12 @@ static bool GetBlocksVersion(string generatorString, out Version version) {
       version = new Version { major = 1, minor = 0 };
       return true;
     }
+    // Likely it's "glTF 1-to-2 Upgrader for Google Blocks"
+    // No other Blocks version info, so assume 1.0
+    if (generatorString.Contains("Google Blocks")) {
+      version = new Version { major = 1, minor = 0 };
+      return true;
+    }
     version = new Version { major = 0, minor = 0 };
     return false;
   }