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/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);
     });
   });