Bug 1029763 - The contact thumbnail scaling is terrible, all jagged

This is a nice simple change and I'm fine with it, provided that:
1) you document it in the comment above

2) in apps/sharedtest/test/unit/image_utils_test.js you modify this line:

```
    runResizeAndCropToCoverTests(size[0], size[1], 'image/jpeg');
```

to add 1.0 or 0.9 or something so that we know the test is exercising the new option.

Was the original bug actually caused by using the default jpeg encoding option? It surprises me that you would have to add this option and use a 1.0 quality setting to get the results you want. I'm not sure what the default value is, but I'm surprised that this makes a visible difference.  Have you compared blob sizes to make sure that a 1.0 jpeg is still smaller than just using a lossless png?