Do not render .hst-cmseditlink overlay button if element is not visible

If an .hst-cmseditlink element is hidden from the user (e.g. display:none or visibility:hidden), the overlay render code will show a button in the top-left of the page at coordinates x=0 and y=0. For example, a site that displays a mobile menu and a desktop menu will most likely have both menus in the DOM at the same time, and show/hide the correct one using CSS breakpoints.

A site can easily work around this by rendering both menus in a single template file (thus having the possibility of using a single .hst-cmseditlink element for both menus), but we should also fix this as it can lead to unintended and weird behavior, e.g. two of these hidden .hst-cmseditlink elements will generate two buttons in the top-left corner, overlapping each other.

It can easily be fixed by checking the width&height of the .hst-cmseditlink element (which is 0 if hidden).