[GFX][FisheyeBindings] Allow principal point to default to center of sensor

[Skylion007]

Motivation and Context

• We currently have to specify the Principal Point which is a big annoyance on the Habitat-Lab side. Most of the time, the principal point should lie in the center of the image. If not, the user can still specify it. Using the new fixed Corrade::Optional bindings to make this a sensible and easy to compute default.

How Has This Been Tested

• PyTest and locally

Types of changes

• Docs change / refactoring / dependency upgrade
• New feature (non-breaking change which adds functionality)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[jturner65]

LGTM

[bigbike]

Check if you need to upgrade the viewer.cpp as well.

[bigbike]

@Skylion007 :
Last thing: check the viewer.cpp.
Then you are good to merge it. :)

Thanks a lot!