Make possible to select remote canvas items inside subviewports

[YeldhamDev]

This is basically attempting to do #99401 but via runtime remote selection. However, I'm having trouble with the transform math, so there are still plenty of issues:

• Selection square is at the wrong location.
• Items can be selected from two different positions (one inside the subviewport, the other relative to the root viewport).
• Mixing both stretch from SubViewportContainer and position/zoom from a Camera2D inside of it results in incorrect click location.

Any help is appreciated.