Image maps project

Implement HTML image map support

Background information: Image maps are an old HTML technology that allows treating arbitrary areas in an image as separate hyperlinks. Servo does not support them yet; the goal of this project is implement this missing feature.

Note: there are tools that can create image maps for testing. The Space Jam website contains a great premade example.

Initial steps:

• compile Servo and ensure that it runs on tests/html/about-mozilla.html
• email the mozilla.dev.servo mailing list (be sure to subscribe first!) introducing your group and asking any necessary questions
• define an Area enum in htmlareaelement.rs that supports rectangles, circles, and polygons (ie. coordinate representations)
• implement constructors for each variant that accept a string argument, parse them into appropriate coordinates, and return an appropriate Area instance
• implement a hit_test method on Area that accepts a Point2D<f32> argument and returns true if the point is within the area's coordinates (return false for all polygonal areas)
• write tests for the Area constructors and hit tests (add a new htmlareaelement.rs to tests/unit/script/ and run ./mach test-unit -p script)
• add a method to HTMLAreaElement that returns an Area value derived from that element's attributes

Subsequent steps:

• add a method named areas to HTMLImageElement that returns a vector of Root<HTMLAreaElement> values - these are the result of finding the HTMLMapElement specified in the image's usemap attribute, then retrieving its child HTMLAreaElement elements
• write a method for HTMLAreaElement that implements the activation behaviour defined in the specification (see HTMLAnchorElement::activation_behavior for a good model)
• implement the handle_event method for HTMLImageElement (as part of VirtualMethods) - if the event is a click event, iterate over the list of associated HTMLAreaElements and hit-test the Area values against the click event's coordinates. If an area matches, invoke the HTMLAreaElement's activation behavior
• extra credit: implement polygon hit testing