create material for texture baking

textures can now be created manually and are no longer tracked by SceneManager (and therefore require manual tracking/disposal)

thermion_dart/test/helpers.dart

@@ -20,6 +20,7 @@ import 'package:path/path.dart' as p;
 Color kWhite = ColorFloat32(4)..setRgba(1.0, 1.0, 1.0, 1.0);
 Color kRed = ColorFloat32(4)..setRgba(1.0, 0.0, 0.0, 1.0);
 Color kGreen = ColorFloat32(4)..setRgba(0.0, 1.0, 0.0, 1.0);
+Color kBlue = ColorFloat32(4)..setRgba(0.0, 0.0, 1.0, 1.0);
 
 /// Test files are run in a variety of ways, find this package root in all.
 ///

thermion_dart/test/texture_tests.dart

@@ -22,6 +22,7 @@ void main() async {
             textureFormat: TextureFormat.RGBA32F);
         await texture.setLinearImage(
             image, PixelDataFormat.RGBA, PixelDataType.FLOAT);
+        await texture.dispose();
       }, bg: kRed);
     });
   });