completely reimplemented renderer::TransformSequence::compute_motion_segment_bbox() using a simpler and more general algorithm based on numerical root finding.  this algorithm handles linear scaling in addition to rotations (translations will be implemented next).  this fixes the massive slowdown reported by Haggi with a scene containing thousands of scaled, moving assemblies.

also added and tweaked unit tests.

what remains to be done:
  - handle translations
  - switch to Newton's method for root finding
  - move all root finding code to a separate module in appleseed.foundation
  - optimize the code and tweak the performance- and precision-related constants
  - clean up the code