Expose original getGutter

TileWMS objects take a gutter option but do not have a public getter
for it. This makes it convoluted for user code to recreate the object
(ex: in the case of serialization/deserialization).

- the getGutterInternal() method is renamed to getGutter;
- the getGutter(projection) method is renamed to getGutterForProjection,
  which is also more explicit.

The getGutter method was not API and is only used by the renderer.

src/ol/source/Tile.js

@@ -151,7 +151,7 @@ class TileSource extends Source {
    * @param {module:ol/proj/Projection} projection Projection.
    * @return {number} Gutter.
    */
-  getGutter(projection) {
+  getGutterForProjection(projection) {
     return 0;
   }