Basic SVG support project

Implement basic SVG support

Background information: Scalable Vector Graphics (SVG) are a vector image format that can be embedded directly in HTML web content (example). Full SVG support in Servo is a huge project; the goal of this project is to implement an extremely small subset in order to explore the capabilities of our experimental WebRender technology.

Note: the scope of this project is encapsulated in https://github.com/servo/servo/issues/12974, https://github.com/servo/servo/issues/12975 and https://github.com/servo/webrender/issues/402

Initial steps:

• compile Servo and ensure that it runs on tests/html/about-mozilla.html
• clone webrender and use a Cargo override to build Servo with the local clone
• email the mozilla.dev.servo mailing list (be sure to subscribe first!) introducing your group and asking any necessary questions
• create the SVGElement DOM interface (guarded by a dom.svg.enabled preference)
• create the SVGCircleElement DOM interface (guarded by a dom.svg.enabled preference)
• make create_element return the new elements when the svg and circle tags are encountered in the SVG namespace
• write a test in tests/wpt/mozilla/tests/mozilla/ showing that the new elements are recognized when the dom.svg.enabled preference is enabled

Subsequent steps:

• implement the graphics primitives necessary for WebRender
• add a SVG type to SpecificFragmentInfo that contains width/height information and a vector of circle data (use the Canvas type as a model, since it is very similar)
• create an instance of SpecicFragmentInfo::SVG in build_fragment_for_block
• add a build_display_list_for_svg_fragment in display_list_builder.rs, and call it from build_fragment_type_specific_display_items TODO