Fix motion blur and temporal denoise in reflections (Motion vector reflection pass)

This task effects people who want accurate temporal denoising and motion blurs in the compositor, no matter how many reflections are used.

The user needs motion passes in order to manually drive the motion vector node, and this technique is also used to clean flickering from animation frames.

--The issue: is that reflection data is not captured, only physical geometry, resulting in obvious visual artefacts (see example). --

**SOLLUTION: Combine a pass which has all reflections of the motion vector masked to glossiness value. **

Example: you had a moving mirror in the middle of the scene, full glossiness would make it opaque in the alpha channel while the rest is transparent. Easy to alpha over existing motion vector data. RGB channels would capture the reflected speed vector of surround geometry as seen through the glossy surface.

The user or the developer can perform some vector maths to combine the two passes to achieve far more accurate inputs for motion blurs. Temporal denoising and any other similar cross-frame based methods will immediately notice more accurate results in reflective scenes.

The below example is based on a guide for temporal denoising in this video: youtube.com/watch

On the left: temporal denoising is applied and it's clear that it does not know there are reflections by the result. It's less subtle, but your reflection motion blurs will have similar issues.

On the right, you can see the original image for reference.