GLMixer Code

Slight behavior change Render view: draw separately the source only if
selected (not current source).

Default mixing function set to opacity in presets.

minor bug fix zoom slider

Updated cmake config to enable validated optional components by default
(ffgl and shapshots now enabled by default).

Improved implementation preview of gamma.

Implemtation of the preview button for color manipulation in mixing
toolbox: allows to briefly disable the manipulation performed to look
the the difference. Improved visual appearance of sliders for color
manipulation (with explicit background).

Implementation of selective Gamma per COLOR components (red, green,
blue) in image processing and associated GUI update in Gamma Levels
widget.

Cleanup glmixer menu.

Added widgets and toolbars layout presets.

Updated masks.