engine: set additive blend only during a fade transition
engine: set additive blend only during a fade transition

When the intensity reaches 1.0, the crossfade can be considered finished, so the original
function blend mode can be restored

mcallegari/qlcplus,nedmech/qlcplus,sbenejam/qlcplus,nedmech/qlcplus,mcallegari/qlcplus,sbenejam/qlcplus,mcallegari/qlcplus,sbenejam/qlcplus,kripton/qlcplus,kripton/qlcplus,sbenejam/qlcplus,kripton/qlcplus,sbenejam/qlcplus,plugz/qlcplus,plugz/qlcplus,nedmech/qlcplus,plugz/qlcplus,mcallegari/qlcplus,plugz/qlcplus,mcallegari/qlcplus,plugz/qlcplus,plugz/qlcplus,mcallegari/qlcplus,kripton/qlcplus,nedmech/qlcplus,kripton/qlcplus,kripton/qlcplus,kripton/qlcplus,sbenejam/qlcplus,plugz/qlcplus,sbenejam/qlcplus,mcallegari/qlcplus,nedmech/qlcplus,nedmech/qlcplus