Fix proxy name lookup by index
vtkSMSessionProxyManager::GetProxyName(const char* groupname, unsigned int idx) was effectively treating the idx parameter as the index into the proxy names. This commit changes idx to instead be an aggregate index into the vectors of proxies associated with the proxy names instead. This is consistent the function vtkSMSessionProxyManager::GetNumberOfProxies(const char* group) when used to iterate over all proxies in a given group name, for instance. This corrects a bug that manifested when loading a ProxyCollection of textures assigned the same name attribute in a state file. This could arise when loading state files from older versions of ParaView that did not ensure that texture names were unique.