Fix ImageContainer2D for Fury v2: wrap image in UI and handle RGB images

[VanshAgarwal24036]

This PR fixes and ports ImageContainer2D to work properly with the current Fury v2 architecture.

Fixes #1108

This PR:

• Wraps the image actor inside a _ImageUI class that properly subclasses UI.
• Implements the required _update_actors_position method.
• Converts RGB images to grayscale to ensure compatibility with the underlying image actor.
• Ensures correct actor positioning using the new UI system.
• Adds a dedicated test for ImageContainer2D.

[VanshAgarwal24036]

Hi maintainers

All tests and ruff checks pass locally.
Kindly approve the workflows so CI can run.

Thank you!

[VanshAgarwal24036]

@maharshi-gor @ganimtron-10 plz review

[m-agour]

Hi @VanshAgarwal24036 , thanks for this PR, but, we are currently develpoing the v2 branch not the master branch, and given there is no conflicts with master I don't think is is based on v2, please always start your branch based on v2 branch not master.

[VanshAgarwal24036]

Thankyou @m-agour I will review again and do changes accordingly to v2 branch