Improve marker generation performance

`getFirstNonCoveredPoint` used to recursively search the rect of an
element until it found a non-covered point. This works well most of the
time, but on some sites (such as prisjakt.se) it took way too much time.

Moreover, this technique required some CSS to be reset in a few special
cases, which is very costly performance-wise. It is also brittle and
makes the code unnecessarily complex.

Lastly, there was a bug in the algorithm that caused uncaught exceptions
sometimes (such as on youtube.com).

Now we use a much simpler approach instead.

`getFirstNonCoveredPoint` tries 6 different points of the element:

    +-------------------------------+
    |1 left-top          right-top 4|
    |                               |
    |2 left-middle    right-middle 5|
    |                               |
    |3 left-bottom    right-bottom 6|
    +-------------------------------+

If all of those are covered (or are reported as covered because of one
of the CSS special cases we used to reset) then the whole element is
simply considered to be covered. This seems to work really well.

The above means that the markers can now be placed at any of the points
in the above figure.

The result is much faster, simpler and more robust.