add startpos method

[gpucce]

Is your feature request related to a problem? Please explain.
It is generally hard to make a translation from an user-unknow (but javis-known) current object position to a specific point.

Describe the solution you'd like
Talking with @Wikunia he came up with a possible interface startpos that should be a function that when called at non-render time returns nothing. The first time it is called at render time returns an object current position, whereas when called again at render time it keeps returning the position of the object at the time it was originally called.

Describe alternatives you've considered
The only alternative I can come up with is a custom action.metafunc that is called before action.func and allows for user defined solutions, though this is really not user-friendly.