Changes to default Gamma values and Warning for old .blend files

[DavidBluecame]

• The default Input Gamma has been changed from 1.8 to 2.2 for Linux and Mac, so now the default input Gamma will be independent from the Operating System.
• As the current default Output Gamma is 1.0, for old .blend files before sRGB was implemented in Blender the Gamma=1.0 will cause wrong results. A warning has been added in case Gamma=1.0 and the Scene/Display device is set to "None". The warning will dissappear if the user changes the Gamma to another value or if he changes the Display device to something different than "None".

[DavidBluecame]

Closing this pull request as Im making other changes as clay material changes and I have to investigate more the different Gamma situations and issues.