Refactored PMREM blur shader code a bit to prevent triggering apparent compiler bug observed on Intel HD Graphics 630 GPU

CatmullRomCurve3.getPoint() should accept an [optional Vector3 target](https://github.com/mrdoob/three.js/blob/master/src/extras/curves/CatmullRomCurve3.js#L104) argument. It's missing in the TypeScript declaration file, which leads to compiler errors if users try to use this option.

Holes are essentially an array of contours. This means they should be Vec2[][] instead of Vec2[].

Function has already been removed from the codebase and is not part of the public API.