Allow for White in set_pixels

[mutesplash]

Missed this when adding White in PR #113

import time
from buildhat import Matrix
matrix_a = Matrix('A')
matrix_a.set_pixels( [[(3,10),(3,5),(3,2)], [(10,10),(10,5),(10,2)], [(7,10),(7,5),(7,2)]] )
time.sleep(5)