Move to Vulkan style function typedefs

PFN_FunctionName is more readable than SCREAMSOUP_T.

Context creation API function typedefs are kept as-is where the original
header provided them, for compatibility and familiarity reasons.

src/x11_init.c

@@ -545,7 +545,7 @@ static GLFWbool initExtensions(void)
     _glfw.x11.x11xcb.handle = dlopen("libX11-xcb.so", RTLD_LAZY | RTLD_GLOBAL);
     if (_glfw.x11.x11xcb.handle)
     {
-        _glfw.x11.x11xcb.XGetXCBConnection = (XGETXCBCONNECTION_T)
+        _glfw.x11.x11xcb.XGetXCBConnection = (PFN_XGetXCBConnection)
             dlsym(_glfw.x11.x11xcb.handle, "XGetXCBConnection");
     }