Adjust for 128x64 display, add #define to choose screen size

[nweeger]

Hello.

Many thanks for the great & useful code :)

For the text demo, I had to adjust things for my 128x64 OLED screen, which too uses the SSD1306 chip.

So submitting the changes back, in case you're interested.

I added 2 #define at the top to select between the sizes.

I can't test on the 128x32 screen, I hope I didn't break anything for it.

[wagiminator]

Thank you! If I find the time, I'll try it out!