Bump pngjs from 3.4.0 to 4.0.0

Bumps [pngjs](https://github.com/lukeapage/pngjs2) from 3.4.0 to 4.0.0.
- [Release notes](https://github.com/lukeapage/pngjs2/releases)
- [Commits](https://github.com/lukeapage/pngjs2/commits)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

package.json

@@ -87,7 +87,7 @@
     "mocha": "7.1.1",
     "ol-mapbox-style": "^6.1.1",
     "pixelmatch": "^5.1.0",
-    "pngjs": "^3.4.0",
+    "pngjs": "^4.0.0",
     "proj4": "2.6.1",
     "puppeteer": "~2.1.0",
     "rollup": "^2.1.0",