common: animation improvements (#393)
common: animation improvements (#393)

* common: don't rely on animation names

This change makes it possible to use SetAnimationByName(nil) to cancel
the current animation. It also saves on map reads when names are not
used.

* common: document animation structs

* common: track animation entities using a map

This simplifies the code a bit. Update order is now random, but that
shouldn't really matter.

paked/engi,EngoEngine/engo,EngoEngine/engo