Motion Blur

## Prerequisites
- [X] I have searched open and closed issues for duplicates.

## Feature
**Is your feature request related to a problem? Please describe.**
This is an improvement and not related to a problem.

**Describe the solution you'd like**
I would like to have the option to enable a sort of 'motion blur' effect to make the behavior of windows look more natural.
I have a 60hz monitor, and when I move windows or move my mouse, the persistence of vision effect breaks down for me.
For example, if I move my mouse at a moderate speed, instead of continuous motion, I see the mouse appear at points along the path of travel.

I would like to request an option for gala to internally render more than 60 frames per second, as many as is practical. It should render them to some sort of accumulation buffer, which is then rendered to my display 60 times per second.

This would make the movement of windows, cursors, etc look more smooth and natural, without being distracting or gimmicky.
This would make use of existing GPU resources currently unused, and would make Elementary look and feel better without an obvious reason why. I think it would just seem more natural and correct.

This effect should not cause any window or object to persist any longer than it presently does, it should simply render more intraframe information to be ultimately presented at 60hz.

**Existing work**
I am not aware of any similar existing work in desktop compositors.

**Describe alternatives you've considered**
An alternative would be to purchase a higher refresh rate monitor, but this would not benefit the majority of users who operate at 60hz. This solution would also work without increasing the bandwidth requirements for my monitor/video mode. I like multiple monitors, but increasing the refresh rate of any monitor reduces the max resolution I can push down a displayport cable, so this would be a clever compromise, I think.

Start by reviewing Gala's compositor and rendering entry points, then compare the requested accumulation-buffer approach with its current 60 Hz presentation behavior. Done would mean an optional motion-blur mode that preserves display bandwidth and does not extend object persistence.

Written by the indexing model from the issue text.