Make animation::animate const

[johannesugb]

Would be cool if there was a const-version of animation::animate because then one instance of animate could be used from different threads in parallel. The current version assigns to mGlobalTransform and therefore, cannot be const. An alternative would be to duplcate instances of aimation and use the duplicated instances from different threads.

Definition of done:

• Either a version of animation::animate that is const has been added, ...
• ... or it has been made sure that animation can be properly copied for the purpose of parallel animations.
• The code is well documented and the Contribution Guidelines have been followed.