I am sorry, but I can't find the version of gfx. I instated it today on the default deposit of fedora 17.
I think that this is a bug :
When I swap the coordinate of x1,y1 with x2,y2 in parameters of thickLineRGBA, I don't get the same result.
Here is a test that show my point :
SDL_Init( SDL_INIT_VIDEO );
SDL_Surface* screen = SDL_SetVideoMode( 400,400, 32,
SDL_HWSURFACE | SDL_DOUBLEBUF );
As you can see I the attachment, I get tow different lines.