Fix issue with relative units in symbols

[LaurenzV]

Fixes #829

I'm not a 100% sure about this fix, so some scrutiny would be appreciated. With that said, I think there were two things going on:

• When resolving relative sizes (like percentages), we only use the width/height of the viewbox, but I think when using a use node, we should use the width/height of that as the reference, if available.
• We did not set the use_size attribute for use nodes linked to symbols.

Let me know what you think.

Given the following SVG:

<svg version="1.1" viewBox="0 0 200 200" xmlns="http://www.w3.org/2000/svg">
    <title>With size on `use` and relative units</title>
    <symbol id="image1">
        <image width="100%" height="100%"
               href="data:image/png;base64,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"></image>
    </symbol>

    <g transform="translate(25 50)">
        <use href="#image1" width="150" height="100"></use>
    </g>

    <!-- image frame -->
    <rect id="frame" x="1" y="1" width="198" height="198" fill="none" stroke="black"/>
</svg>

Before:

After, which also matches Chrome:

[RazrFalcon]

A yet another symbol bug? What a surprise. One the of the worst SVG features.

As for a solution, hard to say. As long as it doesn't break existing tests it should be fine, but it feels like a hack. No idea how it should be handled properly.

[RazrFalcon]

Can we simply override the viewbox size like we do in convert_svg? Then we would not need any changes to the units code. Or does it break something?

[LaurenzV]

Can we simply override the viewbox size like we do in convert_svg? Then we would not need any changes to the units code. Or does it break something?

I guess it should work, let me try.

[LaurenzV]

Hmm, I'm not sure though, because a view box conceptually has a x/y coordinate, too, does it really make sense to just replace the width/height of the view box in this case?

[RazrFalcon]

I feel like it would a more straight-forward approach since we already doing the same with svg.

[LaurenzV]

Better now?

[RazrFalcon]

All good. Thanks!