drawbox: Respect thickness parameter

The drawbox video filter is drawing lines one pixel thinner than
requested. The default thickness is 4 pixel but in fact the lines
drawn by default are only 3 pixel wide.

Change the comparisons in the code to fix this off-by-one bug. Also
change the default thickness from 4 to 3 to minimize the unexpected
changes from the user's perspective.

As I was already touching these lines, I also removed the "maximum" in
the thickness parameter description, as I don't think it was adding
any value and I even found it confusing.

Reviewed-by: Andrey Utkin <andrey.krieger.utkin@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
