Now inherits from ImageEffect and uses a ShaderImageEffect to generate the image. We get to clear duplicate code that was shared between ShaderEffect.gd and ShaderImageEffect.gd this way, and it also uses the undo/redo logic from ImageEffect instead of Canvas.gd.
This comes with the benefit that we no longer need a Viewport node in the ShaderEffect dialog, which lets us have the Preview TextureRect at a fixed size. Previously, the preview was scaled based on the image's size, which would make applying shaders to large images very inconvenient, since the dialog would be way too big.