tried to compute alpha on channel basis

lots of changes in the class computeWeights, could speed up computation
tiny change in comments
only using one loop for computing beta, no arrays
Merge branch 'master' into per_channel_alpha
introduced factor 2 for PER_CHANNEL_ALPHA

yields better results and is taken from sens techreport directly...
Merge branch 'master' into per_channel_alpha
moved RANDOM_PARAMETERS to cpp file

and some typos *cough*
Merge branch 'master' into per_channel_alpha
can now preapply alpha mapping to colors
only doing this with input colors, but still a bad idea => disabled
Merge branch 'per_channel_alpha'