Move the st7920 icon rendering optimizations from display.py to
st7920.py. This simplifies the code for other displays.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
It isn't necessary to use the st7920's xor capabilities when drawing
the bed heating animations. This makes it easier to use the icons for
other displays.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>