works on slider2D, please review and I'll do it for Slider
set white color before rendering background, was too dull
default value for slider on right click
Merge branch 'master' into resetToDefault
Merge branch 'resetToDefault' of https://github.com/nselikoff/Cinder-MinimalUI into resetToDefault
incorporate reset to default into SliderCallback class
Merge pull request #7 from nselikoff/resetToDefault

Reset a slider to its default value with a right mouse click.