GLTFLoader: minor optimization

Description of the problem

https://github.com/mrdoob/three.js/blob/d3bff25fc38c6143665b7ef482f2ebef3073b3af/examples/js/loaders/GLTFLoader.js#L1049-L1052

can be slightly optimized. Also see https://github.com/KhronosGroup/glTF/issues/1411#issuecomment-410356033

Rearranging to

                var s2 = - 2 * ppp + 3 * pp;
		var s3 = ppp - pp;
                var s0 = 1 - s2;
		var s1 = s3 - pp + p;

saves three multiplications during each cubicspline animation frame. Still, not sure if worth it as this may not make a measurable difference.

Three.js version

• Dev

Browser

• All of them
• Chrome
• Firefox
• Internet Explorer

OS

• All of them
• Windows
• macOS
• Linux
• Android
• iOS

[takahirox]

I think this type of optimization should be done by JavaScript Engine. We don’t need to do manually. We should just keep the code simple and readable as much as possible.

Weird that Chrome becomes faster with this change. I know micro benchmark test framework sometimes doesn’t fairly compare the performance. I’ll write the test by myself and share the result.

[donmccurdy]

Tested in Chrome, Firefox, and Safari. It wasn’t consistent across multiple runs. Running the same test in Firefox repeatedly sometimes showed one or the other as faster. Safari was more consistent, with code block 2 being slightly faster. Overall the trend appears to be slightly faster, and intuitively that makes sense, so I’m happy with the PR.