updated diff attached, now both 'my' new modes (avg was around before but was deleted at some point.. <*>) works in plain x11. probably for opengl preview you need to a bit more lines..
finally looked at history..and made this patch (attached)it works softer than logical or porter/duff modes, but notiecably slower ( (on 8 core arm tablet)my modes fails to compile - something about me not really knowing how to write macro functions (?)