Use previous step result as source surface for next step's dialog

[bsneeze]

Some canvas/tool style effects show the EffectSourceSurface (like Smudge and Liquify). Currently, ScriptLab uses the source image, so for example Smudge doesn't show any changes made by previous script steps. This is dependent on fixing #3 first (otherwise users will have to wait for all previous steps to render before a dialog will open)