Offset-plus-delta ticking

[brentfpage]

This PR allows for the tick labels on a given axis to be written as a small change (referred to below and in the code as a delta) relative to an offset. This presentation mode is only entered when the axis range=(max-min) is much less than the axis max and min. Specifically, the most significant digit of the axis range must be 3 decimal places to the right of the most significant digit of max(abs(max),abs(min)).
One thing that has to be considered for the mode is the maximum delta size relative to the range. If the maximum delta size is equal to the range, then the delta constantly cycles through 0 and the offset continually changes as the user pans around, which is hard to follow. On the other hand, if the maximum delta size is 1000 times the range, then it has to be written with ~5 sig figs for reasonable precision (range/100), which can lead to crowding. In the PR, the maximum delta size is configured to be between 10 and 100 times the range, avoiding both these issues.
If the delta is not close to 1, then it would have to be written with a bunch of zeros in the absence of any scaling, which would defeat the purpose of the ticking mode. So, the mode also includes a scaling by 10^(3n) such that the range is between 0.01 and 10 "label units".

The ticking mode in action:

• The default offset-plus-delta presentation, configured using

ImPlot::SetupAxisFormat(ImAxis_X1, ImPlot::Formatter_Offset_Plus_Delta, (void*) "%g");
ImPlot::SetupAxisFormat(ImAxis_Y1, ImPlot::Formatter_Offset_Plus_Delta, (void*) "%g");

:

• With the addition of the ImPlotAxisFlag ImPlotAxisFlags_CompactOffsetAndDelta:

I added this "compact" presentation mode because it's often required on smartphone screens