Optimize AABB calculation in software skinning

[pouleyKetchoupp]

Using min/max instead of regular AABB for optimal processing.
Based on @lawnjelly's suggestion (see #53144 (comment)).

This is 3.x only since software skinning is not implemented on master for now.

[lawnjelly]

It does remind me, at some point we should add a REAL_MAX, that changes to represent the MAX depending on whether real_t is 32 or 64 bit, this would be more correct (although until 64 bit becomes common in Godot 4.x I doubt it matters in practice). Then change all the suitable FLT_MAXs in the same PR.

[akien-mga]

Thanks!