Fix compile time issue in libbirdgems code

The Graphics Gems code was writen by Andrew Glassner and published in
Academic Press, 1990. It is amazing that the code still compiles and
works in 2025. However, the last version off gcc complains about the
function definitions. I updated the function definitions.

Merge branch 'master' of github.com:johanmattssonm/birdfont

Handle debug messages from background threads

Fix compile error in libbirdgems

Create a library of the Graphics Gems code