Support stacked area charts

[fredemmott]

e.g. second chart here:

I have a basic implementation here, but it's very limited - doesn't support the usual flags etc:

• https://github.com/fredemmott/XRFrameTools/blob/11cb058fc9e99636d4810606e23ea298d9666df5/src/ImStackedAreaPlotter.hpp
• https://github.com/fredemmott/XRFrameTools/blob/main/src/ImStackedAreaPlotter.cpp

Usage:

ImGuiStackedAreaPlotter plotter;
plotter.Plot("Foo", plotterFunc, data, count);
plotter.Plot("Bar", plotterFunc, data, count);

Due to #340 , this currently depends on implot internals in order to function correctly when series are hidden.