Bug 1054326: Initial attempt to submit Imagecompare test framework and test cases int...

...o the main branch
I had a thought.. I wonder if this is to do with the device pixel ratio, and that 30 pixels on the device is not the same as 30 pixels in the screenshot. Try multiplying your crop height by `window.devicePixelRatio`, which you can obtain using:
```python
device_pixel_ratio = self.marionette.execute_script(
    'return window.wrappedJSObject.devicePixelRatio;')
```