This method returns an array with three keys: left, right, gnathion.
The value of each of these keys is of the normal Position format described
in the Cloud Vision documentation.
Example:
$positions = $landmarks->chin();
foreach ($positions as $name => $pos) {
echo "Position Type: ". $name . PHP_EOL;
echo "x position: ". $pos['x'] . PHP_EOL;
echo "y position: ". $pos['y'] . PHP_EOL;
echo "z position: ". $pos['z'] . PHP_EOL;
}
