Add LatLngBounds.pad() method

A similar method of the same name was removed in fleaflet#1427.

This method simply expands the LatLngBounds coordinates by
latitude and longitude constants.

Checks to ensure the adjustment remains "in-bounds" is performed,
similar to the extend and extendBounds methods.

lib/src/geo/latlng_bounds.dart

@@ -146,6 +146,15 @@ class LatLngBounds {
  west = max(minLongitude, min(west, bounds.west));
  }
 
+ /// Expands bounding box by [latitude] and [longitude]. This method mutates
+ /// the bounds object on which it is called.
+ void pad(double latitude, double longitude) {
+ north = min(maxLatitude, north + latitude);
+ south = max(minLatitude, south - latitude);
+ east = min(maxLongitude, east + longitude);
+ west = max(minLongitude, west - longitude);
+ }
+
  /// Obtain coordinates of southwest corner of the bounds.
  ///
  /// Instead of using latitude or longitude of the corner, use [south] or