Adding Keystone Correction and Post Projection Scale

[Maxiriton]

Hello,

In this pull request I've implemented vertical and horizontal keystone correction. To do so, I had to change some node in the projection group, but I tried to changed these changes as minimal as possible.

Another feature is the post scale feature. It adds the posibility for users to globally scale the projected image on top of all others parameters.

These changes are done in order to mimic real world projectors. I hope you'll find them usefull.

[Ocupe]

@Maxiriton , thank you for your PR!
I'm going to take some time to review and think about your PR.
I have two questions that might help speed up the process.
I'm curious, have you done research on keystone correction? Are there different implementations? What is the normal range for keystone correction (-1 to 1, -100% to 100%)? For the other settings, I tried to find different resources and then decided on something that made sense to me.

What problem does the post scale feature solve for you? Why not use the aspect ratio? At first glance I would think that this could work against the throw ratio and you could end up simulating a projector that is not physically constrained.
I developed the add-on to simulate projectors and their real-world capabilities, so I think any setting should correspond to a possible real-world setting on a projector. But I'm happy to hear your thoughts.

[Maxiriton]

Hello @Ocupe ,

Sorry for the late answer ! I did these changes upon request from my client who is a director/scenographer and uses Projector for their Theater work.

Here are his anwser :
The post scale is a feature on some beamer, it is expressed in percentage on the covered surface by the original aspect ratio and it is used to help fine adjustments of the beamer on set.

The keystone is an arbitrary value, and each beamer has it owns scale for it, so it goes from -X to +X.

Based on your comments, I think it would be nice to lock the post scale feature in a percentage and not allow a scale that would increase the size of the projected image.
For the keystone parameter, I don't really know how to label it as it is dependent of each beamer.

Thanks,