Improve fps counter

[Meakk]

Describe the bug
The FPS counter is updated after each render, including when only the UI is updated which is not really useful because it reports >1000fps even for heavy models

It should be improved by:

• Updating it only when the whole scene is rendered
• Accumulate it over a few frames to give an average instead of showing the last frame performance only

[GabrielNakamoto]

If this is still open I would like to work on it. I just have one question: in what situations would the whole scene not be rendered?

[mwestphal]

First of all is this still open?

Yes :)

If so I would like to work on it.

I've assigned it to you

I just have one question: in what situations would the whole scene not be rendered?

When we render only the UI, not the 3D scene

[GabrielNakamoto]

PR #1886