I simplified a lot of API calls by pulling panel memory management
inside the library. All API calls which previously required a panel
as parameter are now handling their panel themself.
I removed the default vertex type and instead create an API to allow the
user to specify the vertex struct layout. Of course you are still
bound on what the library actual has on vertex data but the type and
sequence of the data is now configurable. This commit is quite
experiental and could introduce some bugs, but so far it look fine.
