Fixed brightness filter for alpha != 1
Fixed brightness filter for alpha != 1

svn path=/trunk/libavg/; revision=2792

scotty007/libavg,th3infinity/libavg,hoodie/libavg,scotty007/libavg,libavg/libavg,yyd01245/libavg,yyd01245/libavg,hoodie/libavg,scotty007/libavg,libavg/libavg,th3infinity/libavg,th3infinity/libavg,libavg/libavg,yyd01245/libavg,hoodie/libavg,yyd01245/libavg,libavg/libavg,th3infinity/libavg,scotty007/libavg,hoodie/libavg