Revert all the changes in demo.cpp that were never supposed to be there in the last...

[movit] / mix_effect.frag

vec4 FUNCNAME(vec2 tc) {
        vec4 first = INPUT1(tc);
        vec4 second = INPUT2(tc);
        return vec4(PREFIX(strength_first)) * first + vec4(PREFIX(strength_second)) * second;
}