Add low-resolution cleanup to vectorize

Tracing a small raster left stair-stepped edges and soft shadows as opaque rings. The new low-resolution cleanup (on by default for images under 1024px) upscales the input, blurs the pixel steps away and steepens the edges again, with blur and contrast sliders while it runs.

Transparent pixels are snapped at 50% alpha so glows and shadows trace as a single edge rather than a stack of rings, and the applied layer keeps the traced size instead of the source size.