This makes it more efficient to plot piecewise functions by drawing each piece with one renderer, and possible to plot functions with discontinuities by using a renderer to draw each continuous piece.
Contour and isosurface 'auto levels reorg
2D contours: z-min = z-max now makes empty plot instead of infinite loop
3D isosurfaces: d-min = d-max now makes empty plot instead of infinite loop
Fixed contracts and corner case on marching squares and cubes functions
Better selection box and point-label formatting
Increased timeout on extreme-bounds-tests.rkt
