REL-2118: F2 TPE NPE

Drawing the science area at the offset position for Flamingos 2 is failing with a `NullPointerException` because the information it needs to do the drawing has not been initialized.  This is a "how did this ever work" situation fixed by mimicking the behavior of the GMOS science area feature.  Namely, calculating the necessary information before attempting to draw.
One thing that concerns me about the `_calc` method is that almost all of the code seems to be copied and pasted from the `_calc` method in the superclass, `SciAreaFeatureBase`. `GMOS_SciAreaFeature` begins its `_calc` method by invoking `super._calc` and it seems that `Flamingos2_SciAreaFeature` should do the same thing in order to avoid the arbitrary and unnecessary code duplication and to increase maintainability.