support for string parameters in OSL nodes

for now subtype is not defined, but once we start parsing the metadata we can set texture inputs as FILEPATH
also, it takes relative strings and convert to absolute for all strings (which is arguably a good solution, but
should work for now)

intern/cycles/render/graph.cpp

@@ -326,6 +326,7 @@ void ShaderGraph::remove_proxy_nodes(vector<bool>& removed)
 					
 					/* transfer the default input value to the target socket */
 					to->set(input->value);
+					to->set(input->value_string);
 				}
 			}