[4.x] Fix Transform::xform(Plane) functions

[lawnjelly]

The Transform::xform and xform_inv are made safe for Planes when using non-uniform scaling.
Optimization of calling sites to prevent loss of performance from the changes to xform(Plane).

Notes

• 4.x version of Fix Transform::xform(Plane) functions, add Transform unit tests #50637
• No unit tests yet, I'll do those separately, as I think these may be different in 4.x
• Physics changes are similar to 3.x, I can update these in another PR once we have settled on an approach for [3.x] Add Basis helper functions for transforming normals #51354

[akien-mga]

Thanks!