See also flutter/engine#34624

When calling Scene.toImage/toImageSync, we first flatten the scene into a single display list, and then rasterize it on the engine thread. This flattening happens on the Ui thread, unlike regular rendering which happens on the engine/raster thread. The UI thread does not have access to the texture registry necessary to draw textures, so they are not included in the scene.

We should update these APIs so they post a task to the engine thread to flatten with a texture registry.

@dnfield @zanderso