ofxSvg::fixSvgString(): addition to xml preprocessor

[artificiel]

Affinity designer exports "flattened" SVG like this:

<svg width="100%" height="100%" viewBox="0 0 1920 1080" 
version="1.1" xmlns="http://www.w3.org/2000/svg" 
xmlns:xlink="http://www.w3.org/1999/xlink" xml:space="preserve">

this % approach throws of svgtiny and it generates all vertices at (0, 0).

as a reference, the tiger.svg:

<svg version="1.2" baseProfile="tiny" id="svg2" xmlns:svg="http://www.w3.org/2000/svg"
    xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" 
    x="0px" y="0px" width="900px" height="900px"
    viewBox="0 0 900 900" overflow="inherit" xml:space="preserve">

the proposed addition "fixes" this by: (1) checking if a viewBox is defined and (2) checking if width and height are set to 100%. if so, it replaces the 100%'s with the viewBox corresponding values.

(in other words, it should not be a breaking change as an SVG that has width/height defined as values with not be affected, and an SVG without them (or as %) would not work).

It emits warning when operating.

it also makes fixSvgString() static and public, this is to allow it's use outside of an instance, making it possible to slip into a preprocessing step in ofxSvgLoader(). (note that the method was already processing an inout string reference without accessing any member variables, so was not and instance method per se).