Reduce graphics context creation in drawBases()
Reduce graphics context creation in drawBases()

Create the graphics context required by the `drawBases()` method
in the parent function rather than re-creating it in each call
to `drawBases()`.  Graphics context creation is expensive.

igvteam/igv,amwenger/igv,amwenger/igv,igvteam/igv,igvteam/igv,amwenger/igv,igvteam/igv,igvteam/igv,amwenger/igv,amwenger/igv