Circle is easy to find, complicated to work on. Just open libqb.cpp and search “circle”— it’s the first result I found. (Sub__circle, or something similar, is the actual name.)
From memory, first section of code is declarations, followed by boundary checks (make certain the circle is being drawn inside the viewports), followed by the actual plotting routine at the end.
One thing to keep in mind: Circle also plots arcs; not just circles. To adapt the existing code to a fill routine, you’d basically need to plot lines inside that arc routine.
Which, really, I wouldn’t mind seeing a fast arc fill routine to boot. ;)