Added new alpha blending modes & changed ALPHA_ADD
Added new alpha blending modes & changed ALPHA_ADD

New blending modes:
- Subtract (do not take into account source alpha value)
- Multiply (do not take into account source alpha value; does not give good results on a transparent backdrop)
- Maximized (similar to ADD, but with less color saturation)

ADD blending mode has been modified to take into account the source alpha value.
sebavan/Babylon.js,NicolasBuecher/Babylon.js,sebavan/Babylon.js,abow/Babylon.js,jbousquie/Babylon.js,abow/Babylon.js,Temechon/Babylon.js,jbousquie/Babylon.js,sebavan/Babylon.js,NicolasBuecher/Babylon.js,NicolasBuecher/Babylon.js,Temechon/Babylon.js,RaananW/Babylon.js,BabylonJS/Babylon.js,BabylonJS/Babylon.js,abow/Babylon.js,jbousquie/Babylon.js,Kesshi/Babylon.js,Hersir88/Babylon.js,Temechon/Babylon.js,Hersir88/Babylon.js,BabylonJS/Babylon.js,RaananW/Babylon.js,Kesshi/Babylon.js,Hersir88/Babylon.js,RaananW/Babylon.js,Kesshi/Babylon.js