Update Adafruit_GFX.cpp

Drawtime/Char with SPI.setClockDivider(SPI_CLOCK_DIV2) at Adafruit SSD1331:
Size1: old->4.5ms | new->4.5ms  >>> Speedfactor = *1
Size2: old->202ms | new->17.2ms >>> Speedfactor = *11.7
Size3: old->303ms | new->38ms   >>> Speedfactor = *7.9
Size4: old->404ms | new->66ms   >>> Speedfactor = *6.1