Fix cropping centering

[aeirola]

Currently the cropping logic produces slightly off-center icons on very low resolutions. This is especially noticeable on the un-even sizes in iOS notification icons (29px, etc.).

The issue is due to the fact that the icon is first rendered as a low-resolution PNG, from which a certain center section of whole pixels is extracted. Since the cropping needs to be performed on whole pixels, this causes a small "shift" when the input and output resolutions do not match.

Some possible solutions:

• Render the SVG at a higher resolution, crop and downscale
• Modify the SVG viewBox attribute to produce the desired crop directly during SVG rendering

[aeirola]

Here you can see how the first icon is slightly shifted to the bottom right, while the latter is more centered.

Yes, the effect is quite subtle, but still annoying once you see it.