Address review comments for output subcommands

sway/commands/output.c

@@ -35,6 +35,9 @@ struct cmd_results *cmd_output(int argc, char **argv) {
 	config->handler_context.output_config = output;
 
 	while (argc > 0) {
+		config->handler_context.leftovers.argc = 0;
+		config->handler_context.leftovers.argv = NULL;
+
 		if (find_handler(*argv, output_handlers, sizeof(output_handlers))) {
 			error = config_subcommand(argv, argc, output_handlers,
 					sizeof(output_handlers));

sway/commands/output/background.c

@@ -8,7 +8,7 @@
 #include "log.h"
 #include "stringop.h"
 
-static char *bg_options[] = {
+static const char *bg_options[] = {
 	"stretch",
 	"center",
 	"fill",