The new and improved vtkm::cont::ColorTable provides a more feature complete
color table implementation that is modeled after
vtkDiscretizableColorTransferFunction. This class therefore supports different
color spaces ( rgb, lab, hsv, diverging ) and supports execution across all
device adapters.
This commit converts `Canvas` from an abstract class to a concrete
class by converting the pure virtual methods to empty/default
implementations. It provides an implementation for `Canvas::AddLine`
using Bresenham's line drawing algorithm.
