QCPRange: add truncate operation

This is the dual to the existing expand, providing the ability to
restrict a range to lie within another range.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
QCustomPlot: add methods to map QPoint/QRect between pixel and coords

This API assumes the standard practice of using the QCustomPlot xAxis
and yAxis as the horizontal and vertical coordinate spaces,
respectively.

Closes: issue pabigot/qcustomplot#2
Signed-off-by: Peter A. Bigot <pab@pabigot.com>
QCPRange: mark member comparison operators as const

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
QCPAxis: provide access to calculated range for current plottables

Make this available through an API that does not couple its calculation
with a change of the current range setting.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>