ScaleCanvasManipulator: greatly simplified resizing code

[dsizzle]

in preparation for/while researching different resizing algorithms, I found that I could simplify the current resizing code immensely. will try to apply the same change to the normal scale manipulator.

seems to work as well as before, but way less code.

[humdingerb]

As a test, I've saved a stupid picture of me (sweating after a bike ride) and saved as BMP. I then resized from a width of 2988 to 800 pixels with master ArtPaint and this PR build. Put both in a layer and saved as an ArtPaint project: http://0x0.st/o5Kn.artpaint [8.7 MiB]

Switching between those layers, it appears that the new resizing moves the image 1 pixel down and right. Also, if you zoom to 1600% and look at the wall plaster, it appears the old algo has ever so slightly more contrast or sharpness. It probably doesn't matter at all, but I just wanted to mention it.

Tell me if you want to merge as is.

[dsizzle]

I personally can’t see the difference in the wall plaster, which is surprising because I used to play a really dumb game called “I Love Hue” that was all about differentiating subtle color differences. Ah well.

The 1 pixel shift doesn’t bother me. This algorithm is actually bilinear for both enlarge and reduce, whereas the old algorithm is bilinear-ish for enlarge and some sort of weighted average for reduce. I’ll pay the 1 pixel price to use the same code for both.

please merge if there are no other issues, thanks!

[humdingerb]

This algorithm is actually bilinear for both enlarge and reduce, whereas the old algorithm is bilinear-ish for enlarge and some sort of weighted average for reduce.

That may be the reason my eagle eyes see a slight difference. Anyhoo, yay for grabbered bounding box scaling!