A refactor of the margin calculation of UV packing, in anticipation of multiple packing methods soon becoming available.
Three margin calculation options are now available:
* "Simple", the default margin calculation from Blender 2.8 and earlier. [0]
* AABB_LENGTH_SUM margin calculation, the default from Blender 3.3+
* "Fraction", a new (slow) method to precisely specify a fraction of the UV unit square for margin.
The "fraction" code path implements a novel combined search / secant root finding method which exploits domain knowledge to accelerate convergence while remaining robust against bad input.
[0]: Resolves T85978