fix: Support 255,255,255 in pixelmaps as a no-data value

This is marked as a fix rather than a feature as some users were
expecting this behavior.

Author: manthey

src/pixelmapFeature.js

@@ -10,7 +10,8 @@ var util = require('./util');
  * @extends geo.feature.spec
  * @property {string|function|HTMLImageElement} [url] URL of a pixel map or an
  *   HTML Image element.  The rgb data is interpreted as an index of the form
- *   0xbbggrr.  The alpha channel is ignored.
+ *   0xbbggrr.  The alpha channel is ignored.  An index of 0xffffff is treated
+ *   as a no-data value for hit-tests.
  * @property {geo.geoColor|function} [color] The color that should be used
  *   for each data element.  Data elements correspond to the indices in the
  *   pixel map. If an index is larger than the number of data elements, it will

src/webgl/pixelmapFeature.js

@@ -63,6 +63,9 @@ var webgl_pixelmapFeature = function (arg) {
           context.drawImage(img, x, y, 1, 1, 0, 0, 1, 1);
           const pixel = context.getImageData(0, 0, 1, 1).data;
           const idx = pixel[0] + pixel[1] * 256 + pixel[2] * 256 * 256;
+          if (idx === 16777215) {
+            continue;
+          }
           result = {
             index: [idx],
             found: [m_this.data()[idx]]