c_flags_$(build_type) is no longer omitted from build flags.
Quadtree updates.

- Removed requirement that min_cell_size be non-zero in both dims.
- In addition to checking new cell size vs min cell size, we also
make sure that the new midpoint doesn't actually intersect any of
the cell's edges.
Quadtree updates.

- Made union to access child nodes linearly. Closes #10.
Quadtree demo updates.

- Rotation now has a period of 30 sec, up from 10.
- Increased number of points to 200, up from 50.
- Made min cell size 64, up from 10.
- Coverage rects are now filled and bordered.

The slower rotation, greater number of points, and huge cell size
now clearly shows the min coverage rects that would have to be
updated when using the quadtree for dirty rects.