Dxt3 textures are not decoded with the correct Unity texture format.

The Phoenix texture conversion helper maps PxTexture.Format.tex_B8G8R8A8 to TextureFormat.Alpha8, which loses the RGB color channels and makes affected textures render incorrectly.

Expected behavior:
- tex_B8G8R8A8 should convert to TextureFormat.RGBA32.
- The conversion should preserve all color channels for this format.
- Keep the change localized to the texture format mapping logic.

--- Original GitHub issue (auto-fetched) ---
Title: Dxt3 textures aren't uncompressed right

Body:
Currently when we load dxt3 textures, we transform them into uncompressed version.
Within unity we then tell: Format.Alpha8

But checking on documentation, the Phoenix r8g8b8a8 format should be more like rgba32 in unity.

https://docs.unity3d.com/Manual/class-TextureImporterOverride.html

Need to understand and fix it.
