`<text>` support

[jwmcglynn]

Support core SVG text functionality as defined by the SVG2 spec.

• Add SVGTextElement and SVGTSpanElement
• Simple rendering integration for MVP
• Expand to support multiple text spans
• @font-face CSS support
• Support basic CSS attributes font-family, font-size
• Evaluate the full set of CSS attributes to be supported and what is the minimum
• Rendering support with system fonts
• System to register more fonts
• Support per-glyph positioning
• Support all SVGTextElement and SVGTSpanElement methods, including ones requiring glyph layout
• Unicode support
• Resvg test suites

Cleanups

• Add a parser for a space-or-comma separated list for various list-based text attributes. A temporary ParseList function has been added to AttributeParser.cc

Follow-ups

• <textPath> support

[jwmcglynn]

c92e910 adds SVGTextElement and SVGTSpanElement

[jwmcglynn]

82a1d36 adds simple rendering support