Add PNG output option
Detail: The front end adds a new 'PNG output' menu, taking its cues from the Sprite output menu, minus those options that don't make sense for Portable Network Graphics. Internally, the image is processed as though a sprite (this is needed because the output window is rendered by the OS and the OS has no built in PNG support) then converted to PNG on the fly during saving. The choices file is extended to save/recall the output window settings as appropriate. Admin: Submission for PNG bounty.
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