A refactor of the margin calculation of UV packing, in anticipation
of multiple packing methods soon becoming available.
Three margin scaling methods are now available:
* "Add", Simple method, just add the margin. [0]
(The default margin scale from Blender 2.8 and earlier.)
* "Scaled", Use scale of existing UVs to multiply margin.
(The default from Blender 3.3+)
* "Fraction", a new (slow) method to precisely specify
a fraction of the UV unit square for margin. [1]
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
[1]: Resolves T90782
Differential Revision: https://developer.blender.org/D16121
c2256bf7f7