Various changes to correct the positioning of special behaviors. This time the fix is actually performed properly ;-).

levelload.cpp
*) Undid a change I made earlier. I changed << 2 into << 3, but it turns out I was wrong. This seemed to work due to a bug in eventframe.cpp.

bridge.cpp
*) Removed one tile from the default offset value for bridges

event.h, event.cpp, event.cpp
*) Removed the extraOffset variable I added earlier. It turned out this was totally unneeded.
*) Fixed the way X and Y positions in the viewport were determined. It's a lot simpler now, and I wonder why the previous version was so complicated.
*) More changes to get rid of the extraOffset variable.