fix paintScreen() for IDE 1.6.10
fix paintScreen() for IDE 1.6.10

paintScreen() used instruction padding for a delay to allow a byte to be sent
to the display using SPI. This delay became to short with the new gcc version
in IDE 1.6.10

The function has been changed to test for the serial transfer to complete
before sending the next byte.

Arduboy/Arduboy,Arduboy/Arduboy