Webgl / clarified the buffer binding/flushing logic

The Webgl points layer renderer has also been optimized accordingly,
giving out much better performance.

src/ol/renderer/webgl/PointsLayer.js

@@ -323,6 +323,9 @@ class WebGLPointsLayerRenderer extends LayerRenderer {
           baseIndex + 1, baseIndex + 2, baseIndex + 3
         );
       });
+
+      this.helper_.flushBufferData(ARRAY_BUFFER, this.verticesBuffer_);
+      this.helper_.flushBufferData(ELEMENT_ARRAY_BUFFER, this.indicesBuffer_);
     }
 
     // write new data