Nice stuff! But I was confused by the following:

    QOI_RUN8 {
        u8 tag  :  3;   // b010
    ...
    QOI_RUN16 {
        u8 tag  :  3;   // b011

I think there might be a bug lurking in those tag values.

EDIT: Wow, I'm blind. Those 3s were bits consumed by the tag, not tag values...