PixelEditor: Add Background Image Feature for Pixel Art Editor

[ChrisWiGit]

User Story

As a pixel art creator, I want to set a custom image as the background in the editor so that I can use it as a guide to recreate or trace images more easily.

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Acceptance Criteria

1. Background Image Setting:

   • Users can import any image to set as the background within the current editing canvas.

2. Positioning & Resizing:

   • The background image can be moved around the canvas.
   • The background image can be scaled up or down proportionally.

3. Flipping:

   • Users can flip the background image horizontally and vertically.

4. Transparency Adjustment:

   • Users can adjust the transparency/opacity of the background image to better view and compare their pixel art with the guide image.

5. Persistence:

   • The background image settings (position, size, flip state, transparency) persist as long as the user is working on the same project.

6. Non-Destructive:

   • The background image is not part of the actual pixel art and does not interfere with the layers or pixel editing.

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Additional Notes

• Ensure that the feature does not impact performance, especially for large background images.
• Provide intuitive controls (e.g., sliders for transparency, drag handles for resizing, and buttons for flipping).
• Include the ability to reset the background image to its original state if necessary.

[PascalCorpsman]

Implemented with 8d99e3b but with some restrictions.
The Pixeleditor will always load the selected image as "full" background image. If you need to edit the background image, do this in forehand.