Dimensions difference wrong

[aczekajski]

The "diff-y" and "diff-x" from the test info in html report which are taken from the dimensionDifference field of generated config.js, show the difference as positive/negative based on no-idea-what.

For the test where test image is narrower from reference by 2 pixels and longer by 3510px and output is:
diff-x: 2 diff-y: -3510
For the test where test image is also narrower from reference by 2 pixels but shorter by 1864px, it was:
diff-x: -2 diff-y: -1864

I tried to pinpoint the cause, but it looks like dimensionDifference comes directly from node-resemble-js.

Expected behaviour:
Diff values should be consistently negative/positive if the given dimension of test image is smaller/bigger then the reference.

Backstop version: 3.8.8