Work on colorspace and filtering

Hi,
Please find with this pull request some modifications related to colorspaces that, I hope, will help improve duke's colour management.
 
# The notion of a file and a display colorspace which can be different,
# 2 new colorspaces : AlexaLogC and Rec709,
# The color functions have been moved in a new ColorSpace.cpp file,
# Bilinear interpolation is available in the viewer, but not connected to any command/key.

There is also a small routine in DukeMainWindow.cpp that waits for the texture to be loaded in memory before displaying it (only when the mode is unlimitedFPS). I am not totally sure of this code, but without it we only have black images showing up. Please fill free to change/remove this part of code from the pull request. 

 Bests,

C