Graphics buffer: trying to "fade out" an uint8array / best (fastest) way to manipulate pixels

You could also use a 256 element precomputed lookup table I believe:

var lut = new Uint8Array(256);
for (var i=0;i<256,i++) lut[i] = i*0.8;
// ... then in a loop ...
E.mapInPlace(buffer,buffer,lut)