Export PDF's using jsPDF

As promised, here is an example on using the jsPDF library to creating PDF's from an `ol.Map`.

Each commit adds some additional feature to the export:

* The first only adds an example that exports a PDF, similar to the existing PNG example;
* The second commit gives the user the options to select an output size and resolution;
* As increasing the map size and/or resolution requires many more tiles, the third commit ensures these tiles have been loaded before exporting the map;
* The fourth & fifth commit simply restore the example so another export can be made.

I agree that this

  * needs discussion whether we want to expose it
  * needs more docs
  * needs `@api`