gltf: Couple small fixes

- Replace a throwing cast that was meant to be a nulling cast
- ShaderRegex wasn't working for gltfs created with "copy files to gltf"

Change-Id: Id000b015a4e3e22cc84f006fc396572f4084fa53

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

@@ -25,8 +25,11 @@ namespace TiltBrushToolkit {
 public class GltfMaterialConverter {
   private static readonly Regex kTiltBrushMaterialRegex = new Regex(
       @".*([0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12})$");
+  // Matches
+  //    http://...<guid>/shadername.glsl
+  //    <some local file>/.../<guid>-<version>.glsl
   private static readonly Regex kTiltBrushShaderRegex = new Regex(
-      @".*([0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12})/");
+      @".*([0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12})[/-]");
 
   /// <summary>
   /// Information about a Unity material generated from a Gltf node.

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

@@ -1009,8 +1009,7 @@ static Semantic GetTexcoordSemantic(ImportState state, GltfMaterialBase material
       // Thus it needs to be data-driven.
       if (desc == null) {
         // Might happen in gltf2
-        Gltf2Material material2 = (Gltf2Material)material;
-        if (material2 == null) {
+        if (!(material is Gltf2Material material2)) {
           Debug.LogWarning("Unexpected: Non-BrushDescriptor geometry in gltf1");
           return Semantic.Unspecified;
         } else {