Fix issue 13057 copyBits:Fallback:

Fix issue https://pharo.fogbugz.com/f/cases/13057/DominantColor-brokem
Bug is due to broken fallback code when there is no platform specific accelerated BitBlt
Resolution is trivial, 5 bits per color means a 15 bit mask, that is (1<<15-1)hex -> '16r7FFF' rather than 16r3FFF.

I tested that the pharo VM effectively make the symptom disappear, I get this result:
(Form extent: 80@80 depth: 32) fillWhite dominantColor -> NewColor white