Patch for /qr/qr.go

```
Correct rendered image.Image.

Currently the QR code only occupies the upper left 1/c.Scale'th of the Bounds() 
rectangle.
```

Original issue reported on code.google.com by `dic...@rcpt.to` on 14 Jun 2014 at 10:50

Attachments:
- [qr.go.patch](https://storage.googleapis.com/google-code-attachments/rsc/issue-3/comment-0/qr.go.patch)
