Gerber plots: Add initial G01 command to the header,

to ensure linear interpolation mode.
To avoid issues, an interpolation mode must be set before first D01 command.

common/plotters/GERBER_plotter.cpp

@@ -234,6 +234,10 @@ bool GERBER_PLOTTER::StartPlot()
     // Be sure the usual dark polarity is selected:
     fputs( "%LPD*%\n", outputFile );
 
+    // Set initial interpolation mode: always G01 (linear):
+    fputs( "G01*\n", outputFile );
+
+    // Set aperture list starting point:
     fputs( "G04 APERTURE LIST*\n", outputFile );
 
     return true;