Avoid precision errors in TSR

[davidisaaclee]

I'm using the TSR estimator for multitouch gestures that can be arbitrary number of points (something like estimate('tsr', touchesBefore.map(clientPosition), touchesAfter.map(clientPosition))). When doing this with some single-touch point sets, I found that the resulting transform had a === b === 0 – confusing because a single pointer pan should always be a valid translation, no?

I looked into it – it looks like the specific points I was providing were causing a precision error when calculating det. Here's a test that fails on current master: https://github.com/davidisaaclee/nudged/blob/2594af5c79660ef805f8892b20c135151faf03e7/test/estimators/TSR.test.js#L100-L111

Some other valid solutions:

• I could round my inputs to more reasonable precisions
• I could switch to T estimator when I know I have one point

But I'd prefer not to think about that stuff! so here's a PR that fixes this by just switching the order of det expression so that the terms that probably will cancel each other out are next to each other. I don't fully get why it works, but it makes the test pass.

Thanks for the great library, I use it in almost every web app I make.