Signed surfaces clip negative values to zero

[connnnal]

On Windows 10 3.2.5 / 2024.1.1, both the CLIs and libraries in the SDK exhibit this issue. For example, with this file, exporting to BC5 then opening with Renderdoc:

texconv	nvtt
texconv -f BC5_SNORM	nvcompress -bc5s -normal

[NBickford-NV]

Thank you @connnnal ! I can confirm this is a bug in NVTT; I'm going to see if I can get this fixed.