Webgl points / read only one pixel for feature hit detection

Also implements `hasFeatureAtCoordinate`.

`hitTolerance` is not supported for now.

src/ol/renderer/webgl/Layer.js

@@ -3,6 +3,7 @@
  */
 import LayerRenderer from '../Layer.js';
 import WebGLHelper from '../../webgl/Helper.js';
+import {TRUE} from '../../functions.js';
 
 
 /**
@@ -81,6 +82,14 @@ class WebGLLayerRenderer extends LayerRenderer {
   getShaderCompileErrors() {
     return this.helper.getShaderCompileErrors();
   }
+
+  /**
+   * @inheritDoc
+   */
+  hasFeatureAtCoordinate(coordinate, frameState) {
+    const feature = this.forEachFeatureAtCoordinate(coordinate, frameState, 0, TRUE, null);
+    return feature !== undefined;
+  }
 }