app: add option to keep canvas padding in "show all" mode
Add an option to keep the normal canvas padding in "show all" mode, instead of extending the checkerboard pattern indefinitely. This is useful when wanting to show the image content beyond the canvas, while still keeping the focus on the canvas; further commits will extend this mode to behave in more view-related cases as if "show all" wasn't enabled. Add a new 'View -> Padding Color -> Keep Padding in "Show All" Mode" toggle, which controls this behavior, with a corresponding default-value option in the preferences, under "Image Windows -> Appearance".
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