Laser material transformation cache not updated properly

[Mateasek]

The caches of affine transforms in the material of laser segments are not updated properly after change of laser transform.

I will try to describe what I think caused the problem, to have a note in case I end up doing the same mistake. I don't expect anyone to verify.

Intended behaviour

• Laser geometry is specified by LaserProfie and the currently available profiles use segmented beams (i.e. instead of using one long cylinder, the laser cylindrical segments)
• The Laser node stores the laser primitives as its children in the scenegraph
• The laser material caches affine transforms to plasma and laser nodes to speed up observations
• The material of each laser primitive has to be updated after the Laser node transform changes which is done by assigning a new LaserMaterial to every segment
• The reassigning of the LaserMaterials is initiated by the _modified method of Laser node being called as a reaction to transform change by Raysect's notification system
• the _modified method calls the _configure_material method which assigns new materials to every laser primitive.

Possible source or problem

• The caching of transforms in the material uses transform of the laser primitives
• laser primitives are one level lower than the Laser node in the scenegraph (are children of Laser)
• when Laser.transform is change, the notification system is triggered and the notification reaches the Laser._modified method earlier than the notification to the laser primitives to change their to_root transforms
• The result is the transform caches in the new LaserMaterial instances use the deprecated transforms, because the laser primitives were not yet updated.

I hope the explanation is understandable. This can be fixed by dropping and recreating the laser primitives and materials instead of just recreating the materials.

I will upload a fix soon.

[vsnever]

Fixed in #421.