Add support for signed BC4 and BC5

[RunDevelopment]

BC4 and BC5 have signed versions (BC4_SNORM and BC5S) that aren't supported right now. Please add support for them. While the signed formats are less common, they are used nonetheless, so they should be supported.

I have already implemented a BC4_SNORM decoder, so feel free to use my code.

[ScanMountGoat]

Is there a specific file you're trying to decode that doesn't work properly?

[ScanMountGoat]

The snorm variants should still work with image_dds. You may need to remap the value range of the output to convert from unsigned to signed.

[RunDevelopment]

I'm sorry for not being specific enough. I meant request for signed BC4/5 to be added to bcdec_rs.