render background chart correctly for raster charts in single chart mode with rotated viewports

this change corrects a7f724b4a62625cbc4cae5b6e254dd6bd2c39e17
I would like to see the coordinates used somehow reworked involving many more changes.
This would simplify the main render code in chcanv.cpp and glChartCanvas.cpp by changing
the calculations for each chart type.

For example, cm93.cpp:5254 really should not be testing for opengl