[rbrowser] use std::multimap for drawable provider registry
[rbrowser] use std::multimap for drawable provider registry

While nullptr entry may appear several times
olifre/root,karies/root,olifre/root,olifre/root,root-mirror/root,karies/root,karies/root,karies/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,olifre/root,olifre/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,karies/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root