Ensure the render callback is always invoked

[sebastienblor]

Changes proposed in this pull request
This PR changes the ShouldSkipIteration function logic to be UpdateSceneChanges that will return true if the scene has changed. This function was previously returning the opposite value than what it was supposed to do. I'm thus updating the different parts of the code that are calling it.

Also, the previous logic was this if there was nothing updated in the scene, we would return "Converging" instead of calling Render(), but that's not correct. We need this callback to be invoked even if nothing has changed in the scene, as this function just checks the render status and does the necessary if the render has finished, is restarting, etc.... The reason it was working was precisely because the logic in ShouldSkipIteration was inverted, and given the high amount of calls in this function where nothing has changed, we would end up calling Render() to do what's needed.
To clarify, I'm renaming the function UpdateRender and I'm putting everything under a switch of the render status, since that's what this function was supposed to do.

Issues fixed in this pull request
Fixes #2078