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];

test/spec/ol/webgl/rendertarget.test.js

@@ -123,6 +123,26 @@ describe('ol.webgl.RenderTarget', function() {
       expect(spy.callCount).to.eql(2);
     });
 
+    it('returns an array filled with 0 if outside of range', function() {
+      const rt = new WebGLRenderTarget(helper, [4, 4]);
+      helper.createTexture([4, 4], testImage_4x4, rt.getTexture());
+
+      let data = rt.readPixel(-1, 0);
+      expect(data).to.eql([0, 0, 0, 0]);
+
+      data = rt.readPixel(3, -1);
+      expect(data).to.eql([0, 0, 0, 0]);
+
+      data = rt.readPixel(6, 2);
+      expect(data).to.eql([0, 0, 0, 0]);
+
+      data = rt.readPixel(2, 7);
+      expect(data).to.eql([0, 0, 0, 0]);
+
+      data = rt.readPixel(2, 3);
+      expect(data).not.to.eql([0, 0, 0, 0]);
+    });
+
   });
 
 });