There are lots of C++ libraries which will do graphics on multiple platforms eg Fox, FLTK, WxWidgets. Haven't seen any in C. Websites for Fox and FLTK appear to have more questions than answers, so you may be on your own there.
For *nix, X Windows, Motif etc is in C but the books and experts to answer any questions are really difficult to get hold of. I don't know about KDE or Gnome. Qt is C++.
As for parallelism, you're depending on the vendor's API for that processor. Some are scalable, some are not. Again, there are very few experts in these areas. Most people just do it at the start of the project and that is it. If you ask them about it, they'll probably tell you they haven't seen the code since they last debugged it. It is normally the case of "if it works, don't touch it". Debugging parallelism can be quite difficult. You have to be able to tell all processors to stop or wait while you debug one of them.
Make sure you can debug before you venture into those areas.