Improve ternaryPlot.transform() within bounds check

[JulesBlm]

Calling .transform() performs a check if the newly transformed vertices are within bounds of the initial triangle. Occasionally something goes wrong and zooming becomes very jumpy, especially when dragging the transformed triangle near the vertices of the initial triangle.

See this demo

This GIF kind of depict how the check works.
https://twitter.com/FreyaHolmer/status/1300041795670245376