#682 Syntax highlighting overwrites characters

[L.K.P.]

After compiling nedit 5.7 under OpenSuse 42.3, the syntax highlighting with different fonts wasn't usable any more because characters were overwritten on the screen:

Changing the function drawString so that it always clears the area before writing the string seems to fix it. I've attached an appropriate patch for source/textDisp.c. Maybe somebody can look at it and integrate it.