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/osl.cpp

@@ -403,6 +403,9 @@ void OSLCompiler::add(ShaderNode *node, const char *name, bool isfilepath)
 				case SHADER_SOCKET_INT:
 					parameter(param_name.c_str(), (int)input->value.x);
 					break;
+				case SHADER_SOCKET_STRING:
+					parameter(param_name.c_str(), input->value_string);
+					break;
 				case SHADER_SOCKET_CLOSURE:
 					break;
 			}