BrightnessContrast effect brightness handling fixed.
BrightnessContrast effect brightness handling fixed.

Brightness is now adjusted by mixing original pixels
to white or black instead of the previous way which
did not handle transparency correctly.

Change-Id: I3346376cbd79302a67cb1ae5045b4948b1f4115f
Reviewed-by: Kim Gronholm <48e0d40fe5e2fc76008f07e0fdece8a523fc2690@nokia.com>

kobolabs/qtgraphicaleffects