Fixed image size for retina spritesheet icons

dist/images/spritesheet-2x.png has dimensions 540 x 60, but the image was being scaled to 210 x 30.  This was causing toolbar icons to be rendered incorrectly in a retina display.  Corrected scaling to 270 x 30.

```
$ file dist/images/spritesheet-2x.png 
dist/images/spritesheet-2x.png: PNG image data, 540 x 60, 8-bit/color RGBA, non-interlaced
```