Webgl buffer / add utility getSize function

Saves checking whether the buffer was initialized with
an array already

src/ol/webgl/Buffer.js

@@ -95,6 +95,7 @@ class WebGLArrayBuffer {
   }
 
   /**
+   * Will return null if the buffer was not initialized
    * @return {Float32Array|Uint32Array} Array.
    */
   getArray() {
@@ -107,6 +108,14 @@ class WebGLArrayBuffer {
   getUsage() {
     return this.usage;
   }
+
+  /**
+   * Will return 0 if the buffer is not initialized
+   * @return {number} Array size
+   */
+  getSize() {
+    return this.array ? this.array.length : 0;
+  }
 }
 
 /**