New class name for the overlay container

[chnchg]

In the recent commit of reveal.js,

hakimel/reveal.js@fe67bad

the class name of the container is changed to "r-overlay". Please add the new class name additionally to the created container in "chalkboard/plugin.js", so that the latest version of reveal.js will also be supported.

--- a/chalkboard/plugin.js
+++ b/chalkboard/plugin.js
@@ -424,6 +424,7 @@ const initChalkboard = function ( Reveal ) {
                var container = document.createElement( 'div' );
                container.id = drawingCanvas[ id ].id;
                container.classList.add( 'overlay' );
+               container.classList.add( 'r-overlay' );
                container.setAttribute( 'data-prevent-swipe', 'true' );
                container.oncontextmenu = function () {
                        return false; 

[rajgoel]

Can you please create a pull request for this? Thanks!