The gfx/2d contains our abstraction of a typical 2D API (similar to the HTML Canvas API). It has different backends used for different purposes. Direct2D is used for implementing hardware accelerated canvas on Windows. Skia is used for any software drawing needs and Cairo is used for printing.
Previously, Moz2D aimed to be buildable independently from the rest of Gecko but we’ve slipped from this because C++/Gecko don’t have a good mechanism for modularization/dependencies. That being said, we still try to keep the coupling with the rest of Gecko low for hygiene, simplicity and perhaps a more modular future.