PNG and JPG pictures for the icon tag and the import (traced to an outline)

[plazmabokor]

After the icon tag (#225) I tried a logo from the web. It was an SVG with nothing in it but a PNG picture (Inkscape had put the bitmap in as an <image>), and layerling said "The SVG file refers to external files". Most logos people have at hand are PNG or JPG, so I would like layerling to trace a picture itself: the dark part of it (or the light part, for a white logo on black) becomes a closed outline, and from there it goes the way of an SVG.

I would do it in three PRs, so each can be tested on its own:

1. The tracer only, no UI. A new rasterTrace.ts: the pixels of a PNG or JPG in, an SVG with one filled path out. The outline runs between the pixels (marching squares), short straight runs become one line, specks are left out, transparent pixels are never part of the shape, and the line is straightened until it has at most 1200 pieces, so the CAD kernel builds it quickly. No new dependency, about 300 lines, with unit tests (npm test).
2. The icon tag window takes PNG and JPG. Browse... accepts them too, with a small switch for the dark or the light part, and the preview shows the traced outline. Another SVG... on the card the same.
3. The import window takes PNG and JPG. The traced SVG goes through the SVG import that is there, as a sketch body. Guide and changelog.

Before I go on, a few questions, so nothing has to be written twice:

1. Is this something you want in layerling? And is a tracer of our own fine, or would you rather have a library (potrace ports are GPL-2.0, imagetracerjs is public domain)?
2. An SVG that holds only a picture, like the one above: should it be traced too, or only get a clearer message?
3. The icon tag window: is a switch for the dark or the light part enough, or do you also want a slider for where dark turns light?
4. The import: how large should a picture come in? A fixed width (say 50 mm), one pixel as a fixed size (say 0.1 mm), or a width typed in the import window?
5. Is 1200 pieces a good limit? In my test the icon tag built a traced logo of 834 curves, and one of 1618 curves ran into the 30 s timeout of the sketch build.

[henmedia]

@plazmabokor, yes, please, and thank you for asking first.

1. Yes, and a tracer of our own: it keeps the dependencies and the licence simple.
2. An SVG that holds an embedded picture should be traced too; for a link to a file outside the SVG, a clearer message.
3. A switch for the dark or the light part is enough to start with.
4. A fixed width for the import, say 50 mm, with the picture's proportions; it can be resized afterwards, so no extra field in the import window.
5. 1200 pieces is fine.

Three PRs as you describe them suits me.

[henmedia]

@plazmabokor, all three parts are in 1.75.0: the tracer, the icon tag with PNG and JPG, and the import window with pictures 50 mm wide. Thank you for building it in three pieces that each could be tested on its own.

[plazmabokor]

@henmedia, thank you for merging all three and for trying each of them in the editor. Next I'm starting on the Hungarian translation (#141).