Fix scaling not working after resizing the window

[voylin]

Scaling didn't work after resizing the window. In Godot 4, the
SubViewport also has a size changed signal which should be used instead.
This fixes the issue I was having: #698
Tested this in the demo project and my personal project, everything seems to work find again.

The main changes were switching the stretch mode to the new Godot 4 mode Canvas_items like @Calinou suggested. And the viewport change should happen on the SubViewport _size_changed as that one updates later compared to the root viewport, which had more control over the size of the SubViewport for some reason.

Ps.: This is my first pull request ever so I'm not sure if I did a good job explaining everything.

[aaronfranke]

Looks good. I'll squash-and-merge this.

[aaronfranke]

In the future, some suggestions:

• Make a new branch when making a PR.
• Make the title more descriptive (I manually fixed this for you).
• It's better if the PR author is the one that squashes. This is a bit more advanced though.
• Write "Fixes #698" in the PR description so that merging auto-closes the issue. GitHub has some magic words such as "Fixes" and "Closes".

[aaronfranke]

Thanks!

[voylin]

In the future, some suggestions:

• Make a new branch when making a PR.
• Make the title more descriptive (I manually fixed this for you).
• It's better if the PR author is the one that squashes. This is a bit more advanced though.
• Write "Fixes #698" in the PR description so that merging auto-closes the issue.

Thank you for the help and information. I didn't know what squashing means, but a quick google search later and I'm up to date.
I will take these things in mind for next time. ^^