Support instrinsic dimensions for image elements

[bsweeney]

If an image element's width/height is set to auto the values should be based on the image's intrinsic width/height. instead, php-svg-lib attempts to use the keyword as-is which can cause string + numeric calculation errors. If the values are omitted entirely php-svg-lib does not attempt to specify a width/height for the image resulting in incorrect placement.

References:

• https://svgwg.org/svg2-draft/embedded.html
• https://developer.mozilla.org/en-US/docs/Web/SVG/Element/image