Instead of creating a converted declaration each time SetFVF is
called, exactly one declaration is created for each FVF(on demand) and
stored for the lifetime of the device. This avoids memory leaks and
makes keeping track of converted declarations easier. Wether a
declaration is converted from a fvf or not is now a static information
inside the declaration. Those declarations are not destroyed in
VertexDeclaration::Release, they stay for the lifetime of the
device. This keeps us free from tracking the declaration through
stateblocks