Allow zooming on very thin selection boxes

[axelboc]

Fix #1407

I've changed the selection validation logic in SelectToZoom to compute the manahattan distance. I'm no longer converting the selection to a Box and calling #hasMinSize, since Vector3 already provides a method for computing the manhattan distance.

Of course there are always edge cases to deal with and unforeseen consequences...

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The edge case is zooming on a 0-width or 0-height box. To deal with this, I had a few options:

1. Keep a hasMinSize(1) check in the validate prop.
   • This kinda goes against the premise of There is no way to zoom on a small slice #1407 that if the user releases the pointer, something should happen.
   • It also meant that we could see a dashed line when crossing the 0 width/height threshold (because that's the style of an invalid selection box). This style made more sense when the invalidity threshold was wider.
2. Ensure the zoom box can never be empty via the transform prop.
   • This would have made the code of computeZoomSelection a bit messy.
3. Ensure we never zoom on an empty box by enforcing a minimum zoom width/height of 1.
   • Seemed like the best compromise, so I went with this solution.

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The unforeseen consequence had to do with SvgRect:

We work around the fact that SVG doesn't support drawing inside strokes by drawing a slightly smaller and slightly offset rectangle. The problem is that empty rectangles are simply hidden by browsers, so any SvgRect with a width or height lower than or equal to their stroke width would simply vanish.

This behaviour wasn't too problematic when we couldn't zoom on a very thin box, since at the point the selection box disappeared, the zoom box was invalid — so releasing the mouse did nothing. So I had to find a fix.

I decided to replace rect with path, which seems to work great, fortunately.

[loichuder]

Congrats for the smart workarounds, this is great 👍