Add Placeholder for Missing Textures in JDMod

[mophawka]

JDMod should scan for all JD22 textures and, if they are not found in other games or songs, automatically replace them with a placeholder image. This will make it easier to identify which textures need to be manually replaced or created before export.

Benefits:
Streamlines the workflow by narrowing down missing assets.
Simplifies the process of identifying gaps in the textures.
Prepares for future builds where missing items can be implemented automatically.

Possible Implementation:
JDMod identifies missing textures.
If a texture is missing, a placeholder image (e.g., a white image) is used as a default.
Placeholder images can be standardized across similar assets (e.g., all mapbkg textures use the same placeholder).
A hash-based file matching system can later help pinpoint missing files easily.

Example:
All map_bkg.tga.ckd textures are replaced with a simple white placeholder image, allowing users to quickly see and address missing textures.

This feature will greatly assist in refining exported games and ensuring no missing assets go unnoticed.

Thanks!