Heltec.display->drawString does not clean the Pixel buffer

[antonmeyer]

Heltec.display->drawString I´m not sure if this is by design. I would name it a bug. When I want to write a string to a given position I do not want to think about what pixels have been there before. So please clean the pixel buffer before writting the new character to the given postition or overwrite the pixel but do not do an OR. In most cases it does not make sense in text mode. or provide a clear line function.

I can use clear, but I do not want to draw the whole screen for every update.