Only update the cursor selection once per mouse-down

In Chrome, right-clicking repeatedly generates both a contextmenu AND
a mouseup event for subsequent clicks. E.g. a sequence of right clicks
generates:
1st - mousedown, contextmenu
2nd...nth - mousedown, contextmenu, mouseup

This causes problems when clicking on images as the selection is
immediately reset after being processed by the first context menu,
causing the subsequent mouseup event to try and reset the cursor
selection *again*.