Refactoring - Combine decodeRow(1,4,8) functions
Support reading images with a bitcount of 2
Formatting
Support OS/2v1 images with less than a full-sized palette
Support most uncompressed OS/2v2 BMPs
Tolerate out-of-range palette indices
Don't bother to read palette colors that won't be used
DecodeConfig(): Read and return the palette

The Go standard library has been inconsistent as to whether DecodeConfig
returns the palette or not, but now it seems to have settled on returning it.