Mirrors now use a singleton to reference the main camera to make changing the camera and instancing mirrors easier

[EhoNo1]

It felt a little inefficient to have to manually assign the main camera to every individual mirror. I found out you can dynamically add and remove autoload/singletons in plugin startup (link) and so I gave it a shot.

Weirdly, Godot seems to fail to detect the plugin's singleton in the editor, but the mirror still appears to work perfectly in the game. This does seem to be a known issue though. I made a small one if-statement workaround, no idea if I'm missing something here.

Let me know if you have any suggestions.

[EhoNo1]

Closed due to lack of interest and possibly poor etiquette on my part. You're welcome to reopen it though.

[Norodix]

Hey! Sorry for not interacting, there was no poor etiquette or anything like it.
I had little time and every time I looked at this PR it was a bit confusing for me, because I cannot decide if it would be better or not to assign cameras in a singleton.
It seems like the simpler and better approach but I'm a bit reluctant to reduce the level of control the user has. I can't think of a use case for assigning different main cameras though.
So I'm a bit undecided.