Add a type alias to goog.vec.Number

This addition is related to the definition of olx.FrameState in olx.js. Without this alias and corresponding goog.provide in src/ol/vec/mat4.js we get this error when compiling an OpenLayers application:

ERR! compile node_modules/openlayers/externs/olx.js:6853: ERROR - Bad type annotation. Unknown type goog.vec.Mat4.Number
ERR! compile  *     coordinateToPixelMatrix: goog.vec.Mat4.Number,

Currently, the Closure Library does not have a goog.provide for goog.vec.Mat4.Number. If it did have this goog.provide then we wouldn't have the error.

This commit works around the issue by creating our own type alias (ol.vec.Mat4.Number) and adding a goog.provide for that type alias.

externs/olx.js

@@ -6850,7 +6850,7 @@ olx.view.FitGeometryOptions.prototype.maxZoom;
 /**
  * @typedef {{animate: boolean,
  *     attributions: Object.<string, ol.Attribution>,
- *     coordinateToPixelMatrix: goog.vec.Mat4.Number,
+ *     coordinateToPixelMatrix: ol.vec.Mat4.Number,
  *     extent: (null|ol.Extent),
  *     focus: ol.Coordinate,
  *     index: number,
@@ -6858,7 +6858,7 @@ olx.view.FitGeometryOptions.prototype.maxZoom;
  *     layerStatesArray: Array.<ol.layer.LayerState>,
  *     logos: Object.<string, string>,
  *     pixelRatio: number,
- *     pixelToCoordinateMatrix: goog.vec.Mat4.Number,
+ *     pixelToCoordinateMatrix: ol.vec.Mat4.Number,
  *     postRenderFunctions: Array.<ol.PostRenderFunction>,
  *     size: ol.Size,
  *     skippedFeatureUids: Object.<string, boolean>,

src/ol/vec/mat4.js

@@ -1,8 +1,16 @@
 goog.provide('ol.vec.Mat4');
+goog.provide('ol.vec.Mat4.Number');
 
 goog.require('goog.vec.Mat4');
 
 
+/**
+ * A alias for the goog.vec.Number type.
+ * @typedef {goog.vec.Number}
+ */
+ol.vec.Mat4.Number;
+
+
 /**
  * @param {!goog.vec.Mat4.Number} mat Matrix.
  * @param {number} translateX1 Translate X1.