Initial fix for #3975, label engine vectorizing texts in SVG and PDF output

- Add labeling engine option to render text-as-text
- Default is still text-as-outlines (vectorized), due to differences between text (as text) and buffer (as outline) methods
- Good output with printing to PDF (searchable, selectable text and embedded fonts)
- OK output with SVG, but differences between text (as text) and buffer (as outline) methods

Does not yet include unit tests or auto-setting of text-as-text for SVG output