u8 encoding and decoding for signed formats

[ScanMountGoat]

The current behavior results in unexpected results after decoding again for bc4s and bc5s. This behavior should be compared with different GPU hardware and programs. Some implementations seem to use a different default value for the blue channel when decoding bc5s compared to bc5, for example.

[ScanMountGoat]

The convention in software and hardware implementations is to remap the -1 to 1 range to 0 to 1 when converting from snorm to unorm. This makes all values displayable and preserves the underlying data for encoding and decoding.