Fix palettized GVR texture roundtrip

[HeitorSpectre]

What changed

• export palettized GVR textures to indexed TGA files with the correct alpha descriptor
• preserve palette metadata so edited true-color TGAs can be reinserted without losing the original palette
• add a regression test covering indexed TGA export header handling

Why

Resident Evil: Code Veronica textures with transparency were breaking during the extract/edit/reinsert workflow. Edited TGAs were often saved as true-color images, which previously caused reinsertion to lose palette data or rebuild the wrong palette.