Fix output config retrieval for new outputs

This removes `output_find_config`, which would take the first matching
output config it found. This is fine if only a name output config,
identifier output config, or even just wildcard exist, but if there is
a name output config and identifier output config, they are not merged.
Instead, this introduces find_output_config, which is just a wrapper
for `get_output_config`. This ensures that both the name and identifier
output configs are respected.

This fixes the following case:
- For simplicity in this example, remove all output configs from config
- Run `swaymsg output <name> bg #ff0000 solid_color`
- Run `swaymsg output <identifier> scale 2`
- Disconnect and reconnect output

Without this, the output will have the background, but not the scale.
With this, the output will have both the background and scale

sway/config/output.c

@@ -438,6 +438,12 @@ static struct output_config *get_output_config(char *identifier,
 	return result;
 }
 
+struct output_config *find_output_config(struct sway_output *output) {
+	char id[128];
+	output_get_identifier(id, sizeof(id), output);
+	return get_output_config(id, output);
+}
+
 void apply_output_config_to_outputs(struct output_config *oc) {
 	// Try to find the output container and apply configuration now. If
 	// this is during startup then there will be no container and config