#589: 2D panels transparent at night. May also fix #591.

#589: 2D panels transparent at night. May also fix #591.
Since d91a617423fa5c269902a847494b3778b05ebcb4 (vectory color calculations)
all 4 elements of a color were scaled with the respective brightness.
The caused some panels to be transparent at night etc.
Before the change, only the 3 RGB elements of the colors were scaled,
while the 4th element (alpha channel?) remained unaltered.
Commit restores the alpha elements to their original behaviour.
=> "Quick hack" for 2.6.0. Need s.o. to double check the initial changes
and commit something clean to "next".