Preview post-processing stage changes in appleseed.studio

[laurelkeys]

This adds a rendered preview when parameter values are changed for post-processing stages (similar to the current behavior of False Colors).

Note that only the effect that is being changed is applied for preview (i.e. if there are multiple stages, this won't show all of them).

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Obs.: as of right now, when you add a new effect it doesn't work untill you click OK to add it and then edit its parameters. This should be fixed before this PR is merged.

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Other "nice-to-have"s are:

• Render effects preview on a downscaled version of the frame, so it's more responsive (and make this a customizable option)
• Some way to "opt-out" of this (e.g. a checkbox), either for all stages or for a specific one, untill all effects are "real-time"
• Save / export the current frame, with effects preview (should probably be an optional "pop-up")

[oktomus]

Franz and I just tested your PR. Amazing work !

We are concerned about the processing time taken by the bloom effect or other future resource-intensive effect. I think we already discussed about this, but maybe you can add a checkbox to control the preview ? Like the following :

That checkbox won't be visible when first adding the effect, as the preview doesn't work there.