[rt #83212] avoid a possible bad optimizer on centos 5.9

[rt #83212] avoid a possible bad optimizer on centos 5.9

rotate(degrees => 270) was producing glitches near the horizontal and
vertical centrelines only on 5.9 with the default (perl config
specified) optimizations.  These disappeared when Imager was built with
-O1 instead.

Re-arranged some code to avoid duplicate floor() and double -> long
conversions which seems to have avoided the optimization problem.