Bug 972666 - [Calendar] lines over events longer than 2 hours r=gaye

I reverted the commit that introduced the bug and added a "hack" to avoid the [performance issues caused by z-index + APZ](https://bugzilla.mozilla.org/show_bug.cgi?id=972675)

now performance should be similar on week and day views, with and without events. tried to change as few lines of code as possible without causing undesired side-effects.