Draft: Fix yelp color on big endian

Fixes: #61

This is a draft version, because it give the good color for yelp on big endian system, but at the same time it breaks a lot of unit tests. I'm wondering if the unit tests are wrong too on big endian, or if it's the right location to fix this.

Also some macros https://gitlab.freedesktop.org/pixman/pixman/-/blob/pixman-0.40.0/pixman/pixman-combine32.h#L5 don't have a special case for big endian, which looks suspicious.