`ImageShader` should have a `dispose` method, and it should be called by the framework.

Image Shaders, particularly those that end up creating an `SkImageShader`, can hold substantial amounts of memory.

As we wire up methods to make sure we dispose of `Layer` objects and related memory, we should make sure we dispose of these as well.

see also #81514, https://github.com/flutter/flutter/issues/57746