Fix image size for svg

[nighca]

Updated behavior: Use the viewBox size for SVG images in Builder to ensure consistency with SPX.

Previous behavior: We retrieved the size of the <img> element containing SVG to determine the image size. While it was rendered within a 300x150 box by default, resulting in dimensions like 300x150, 140x150, 300x100, etc.

Why is the rendered SVG sometimes very small?

In some user-agents, when they fail to calculate a size for the element they fall back to a default size for replaced elements of 300px width × 150px height. That size is from deep within a dark corner of the CSS 2.1 spec, so I guess that’s somewhat reasonable. Mostly it seems to happen when the viewBox attribute values are set in pixels instead of unitless.

Related:

• https://github.com/goplus/spx/blob/15b2e572746f3aaea519c2d9c0027188b50b62c8/internal/svgr/svg.go#L39
• https://thatemil.com/blog/2014/04/06/intrinsic-sizing-of-svg-in-responsive-web-design/

[qiniu-prow]

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[qiniu-ci]

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