removed rounding of values in composeSMaskLuminosity

maskData comes out of maskCtx.getImageData, so is 0..255 clamped, and
the used multiplications will not create fractions needing rounding,
neither would addition.

src/display/canvas.js

@@ -639,9 +639,9 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
   function composeSMaskLuminosity(maskData, layerData) {
     var length = maskData.length;
     for (var i = 3; i < length; i += 4) {
-      var y = ((maskData[i - 3] * 77) +     // * 0.3 / 255 * 0x10000
+      var y = (maskData[i - 3] * 77) +  // * 0.3 / 255 * 0x10000
-               (maskData[i - 2] * 152) +    // * 0.59 ....
+              (maskData[i - 2] * 152) + // * 0.59 ....
-               (maskData[i - 1] * 28)) | 0; // * 0.11 ....
+              (maskData[i - 1] * 28);   // * 0.11 ....
       layerData[i] = (layerData[i] * y) >> 16;
     }
   }