Webgl / return 0 if doing renderTarget.read outside of data width/height

This would happen when WebGLPointsLayerRenderer.forEachFeatureAtCoordinate
is called on "warped" worlds at -360/+360 degrees, and may produce false
positives.

src/ol/webgl/RenderTarget.js

@@ -115,12 +115,18 @@ class WebGLRenderTarget {
   /**
    * Reads one pixel of the frame buffer as an array of r, g, b, a components
    * in the 0-255 range (unsigned byte).
+   * If x and/or y are outside of existing data, an array filled with 0 is returned.
    * @param {number} x Pixel coordinate
    * @param {number} y Pixel coordinate
    * @returns {Uint8Array} Integer array with one color value (4 components)
    * @api
    */
   readPixel(x, y) {
+    if (x < 0 || y < 0 || x > this.size_[0] || y >= this.size_[1]) {
+      tmpArray4[0] = tmpArray4[1] = tmpArray4[2] = tmpArray4[3] = 0;
+      return tmpArray4;
+    }
+
     this.readAll();
     const index = Math.floor(x) + (this.size_[1] - Math.floor(y) - 1) * this.size_[0];
     tmpArray4[0] = this.data_[index * 4];