feat: Render referenced elements in use tags

[theminecoder]

<use> elements lists to reference another element by id. This enables the codebase to render such element when rasterising.

I am aware this is not completely to spec in regards to x,y,width,height but this does the base job for when transforms are at play, which is what my current case exposes. I may look into that in another pr

[meyfa]

Thanks for the contribution! I've left some ideas to further improve this.

The most important thing I would like to have is a check for cycles. Currently, the <use> tag could even reference itself, or reference a parent node, which would cause infinite recursion.

What makes this complicated are cases like:

<use id="foo" href="#bar" />
<use id="bar" href="#foo" />

One solution I can think of would be:

• Store an array of SVGUse elements on the SVGRasterizer (initially empty)
• At the beginning of SVGUse::rasterize(), check if $this is part of the array; if yes, return; if no, add it to the array, and continue as usual.
• At the end of SVGUse::rasterize(), remove $this from the array.

[meyfa]

Please remove this doc comment

Suggested change

/** @var SVGDocumentFragment $root */

[meyfa]

1. Please avoid reusing variables, especially when the type changes ($element was string before this line, but is now a node object). Maybe rename the string variable to $href to reduce confusion.
2. strpos($element, "#") + 1 ?: 0 is problematic - if the string is not found, strpos returns false, and false + 1 makes little sense.

However, it seems to me that the href always has to start with #, and so this can (and should) be rewritten:

• At the start of the function, if $href does not start with #, return.
• Otherwise, remove the first character, and use that for getElementById.

Sources: https://www.w3.org/TR/SVG11/struct.html#UseElement