Detected a display bug when looking for possible causes of

Detected a display bug when looking for possible causes of
bug 1248. Widhts were only calculated in teacher mode and
they were down to 100px in student mode!!
Perhaps we could make some session caching to avoid some
calculations every time the course is displayed. Seems to
work fine now.
Anyway, if I compare the right column width
in my server (with blocks running) against moodle.org (without
blocks), it seems that the right column is narrower in my test server...