Move the fixed size ensuring size to the end of window setup

[andydotxyz]

Fixes order of operation issues such as #2819

Fixes #2819

Checklist:

• Tests included. <- subtlety in render size for some Linux WM
• Lint and formatter run with no errors.
• Tests all pass.

[andydotxyz]

Tagged @Bluebugs on this as I think you worked on this recently

[Bluebugs]

This sadly doesn't fix @kheyse-oqton problem in #2800. It is still a progress as I get a 0x0 window (and crash) with master and the provided example. I think it is an issue where the proper size is only really known after the windows has been built.

[andydotxyz]

I removed #2800 from the fix list earlier today, this remains just a fix for #2819 as the description states above.
Can it be reviewed on the merits of that regression fix please?

[Bluebugs]

I am a bit uneasy that it doesn't fix #2800 as this patch change the behavior in that issue, but I can see that it does improve the situation. So let's move forward with this.