Bit of cleanup

tools/editor/common/dtf.js

@@ -144,10 +144,12 @@ const DTF = (() => {
     for (let i = 0; i < width * height; i++) {
       const o = i * 4;
       switch (format) {
-        case FORMAT_ALPHA:
-          out[o] = out[o + 1] = out[o + 2] = redAsAlpha ? src[o] : src[o + 3];
-          out[o + 3] = 255;
+        case FORMAT_ALPHA: {
+          const v = redAsAlpha ? src[o] : src[o + 3];
+          out[o] = out[o + 1] = out[o + 2] = v;
+          out[o + 3] = v; // use value as canvas alpha so background shows through transparent areas
           break;
+        }
         case FORMAT_RGB:
           out[o]     = src[o];
           out[o + 1] = src[o + 1];