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/renderer/webgl/TileLayer.js

@@ -161,7 +161,7 @@ class WebGLTileLayerRenderer extends WebGLLayerRenderer {
     const pixelRatio = tilePixelSize[0] /
         toSize(tileGrid.getTileSize(z), this.tmpSize_)[0];
     const tilePixelResolution = tileResolution / pixelRatio;
-    const tileGutter = tileSource.getTilePixelRatio(pixelRatio) * tileSource.getGutter(projection);
+    const tileGutter = tileSource.getTilePixelRatio(pixelRatio) * tileSource.getGutterForProjection(projection);
 
     const center = viewState.center;
     const extent = frameState.extent;