lua-imlib2: fix memory leak

[simotek]

If you frequently use this function to draw new images this leak becomes quite significant. I use this function for a "Photo Frame" that shows a new photo every 30 seconds, without this change the image was leaked once each time, with this change that leak is resolved.

Checklist

• [ X] I have described the changes
• [X ] I have linked to any relevant GitHub issues, if applicable
• [ X] Documentation in doc/ has been updated
• [X ] All new code is licensed under GPLv3

Description

• Describe the changes, why they were necessary, etc
• Describe how the changes will affect existing behaviour.
• Describe how you tested and validated your changes.
• Include any relevant screenshots/evidence demonstrating that the changes work and have been tested.

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