Balance smask process chunking

src/display/canvas.js

@@ -652,7 +652,8 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
     }
 
     // processing image in chunks to save memory
-    var chunkSize = 16;
+    var PIXELS_TO_PROCESS = 65536;
+    var chunkSize = Math.min(height, Math.ceil(PIXELS_TO_PROCESS / width));
     for (var row = 0; row < height; row += chunkSize) {
       var chunkHeight = Math.min(chunkSize, height - row);
       var maskData = maskCtx.getImageData(0, row, width, chunkHeight);