v0.19.1

Bug fixes:

• jab/jch color spaces: adjust the PQ transfer function to produce a value of 0 instead of NaN when invoked with a negative value.
• improvements to clampChroma(): use the appropriate stop condition for the bisection search based on the range of the Chroma for a particular color space; when chroma: 0 is not displayable return the RGB clamping of the achromatic version, which improves the function's perceptual continuity.