Adds a graphics subsystem plus a BGA driver to provide a default renderer. Since the BGA backend has to perform calculations every time it's going to find a pixel, it's fairly slow at rendering, but it's decent enough.

Right now, graphics can be loaded with runtime.graphics.enableGraphics() and pixels can be manipulated by fetching them through runtime.graphics.screen.get(x, y) which will return a pixel from the backend. The backend should return a pixel with getters and setters for r, g, and b.

This is basically as thin as possible of a wrapper around multiple possible graphics renderers (BGA, virtio-gpu in the future), to be able to abstract direct access to the video buffers, providing the most basic functions (enabling graphics, pixel manipulation) allowing higher level functions (image drawing, printing text, shape drawing, etc.) to be implemented on top of that.

However, this should be refactored into an optional module later, along with other things.