misc: change storage size of image slices

[haganbmj]

I can do a dumb change to double the desired size when the Slice Window loads, but that has a couple drawbacks.

• It enlarges the slice window pane, which doesn't have a size limit and doesn't attempt to constrain the preview.
• It unnecessarily upscales small images to 200% leading to (slightly) increased size when stored.
• I don't like having a static scale to save with, it really could just look to store "as large as possible." This would help with any future exports at any size and help the issue of storing larger than source - though that plays into the next problem regardless...
• MSE does visual transformations (like sharpening) on the image before it's stored, so this is done at whatever scale/resolution is used for storage. This changes behavior as a 100% export is now going to downscale its sharpen. Which brings up the point that that's already happening at lower/higher resolutions in MSE anyways... Maybe this just needs to be a config option since I don't want to fix when the effects are applied.

[haganbmj]

Alright, so updated this to just put the image scaling selection on the user. Ultimately I think this needs a better solution because storing over-sized images seems a bit silly, but I guess reapplying all filters later is equally terrible.

Also put in a mediocre "solution" for the slice window by just constraining both the preview and slice images. This at least means some consistency with how styles load, but I expect the default is too small and should provide better support for resizing the window.