Offscreen canvas improvements project

Background information: Major browsers support the OffscreenCanvas standard which can be used to create drawing canvases that are not tied to a HTML canvas element visible on the page. Servo is a new, experimental browser that supports some of the offscreen canvas API; the goal of this project is fix several bugs known to cause significant test failures as well as add some missing features.

Tracking issue: (please ask questions in these issues)

• https://github.com/servo/servo/issues/24271 (getting pixel data returns blank pixels)
• https://github.com/servo/servo/issues/24269 (support drawing offscreen canvases onto other canvases)
• https://github.com/servo/servo/issues/24272 (support converting offscreen canvas data into a blob)
• https://github.com/servo/servo/issues/24273 (implement linking offscreen canvases to placeholder canvases)

Useful references:

• Guide to getting Servo building
• Documentation for types and modules inside Servo

Initial steps:

• email the mozilla.dev.servo mailing list (be sure to subscribe to it first!) introducing your group and asking any necessary questions
• fix the bug with getting image data out of offscreen canvases to obtain more meaningful test results
• add support for drawing offscreen canvases to other canvases

Subsequent steps:

• implement blob conversion support
• implement linked placeholder canvases