PixelPaint: Allow resizing images and layers.

[jpgleeson]

Fixes #11862 This PR adds a None ScalingMode to LibGfx and incorporates that into PixelPaint to allow for resizing of the canvas without scaling the image. The previous behaviour was to always rescale the canvas when resizing an image. This adds a radio button to the resize image dialog, which blits the existing canvas at it's existing size to the top left of the new, resized canvas.

[BuggieBot]

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[AtkinsSJ]

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