Simple foreignObject support and viewBox fix

• Copy foreignObject elements from the source SVG to the sketched SVG. These elements have been discarded previously but foreignObjects are sometimes used for text labels in SVGs (e.g. mermaid diagrams), so now they should at least appear in the SVG output. Note: This is currently only supported for SVG rendering.
• Fixed default scaling behavior if viewBox differs from the viewport of the SVG. For preserveAspectRatio only none and the default (i.e. xMidYMid) behavior is supported.