Remove the use of Math.round() in the SSIM diff generation function

[omnisip]

Part 1 of 2 issues that are big performance improvements for the library.

Recently, I built a new implementation of SSIM and had it incorporated into SSIM.js (weberSsim). During testing and implementation, I found that the use of Math.round and Math.pow (also seen as the operator** in ES6) had horrible performance times. It was so bad that I found that I could increase performance by 66% and, in the test harness 60-80ms per test, by removing the use of Math.round and replacing it with a reusable Int32Array and adding floating point 0.5 (matching the Math.round specification) before saving. In our case, this would greatly improve diff generation performance for SSIM based testing.

As such, I would like to submit a pull request with these changes so this library can benefit too.

[github-actions]

This issue is stale because it has been open 30 days with no activity.