--camera-index should override other camera options

[mwestphal]

Describe the bug
F3D has many camera option but there is one that rely on camera provided by the file, however it should override other camera options but it does not

To Reproduce
Steps to reproduce the behavior:

1. Open the file using f3d --dry-run Cameras.gltf --camera-index=0 --camera-position=0,0,0
2. Camera should be positioned correctly but it is not

Expected behavior
Camera positioned as if camera-position was not provided

Additional context
In F3DStarter.cxx:1075, SetupCamera is always called. It should only be called if camera-index has no value, as its an std::optional

Also a test should be added.

[dk1242]

Hi,
I want to work on this issue.

[mwestphal]

Of course, I just assigned it to you