Fixed initial window position and size for hidden windows

As described in the documentation for *glfwSetWindowPos*:

> X11: Some window managers ignore the set position of hidden (i.e. unmapped) windows, instead placing them
> where it thinks is appropriate once they are shown.

This behaviour can be changed with the *PPosition*, *USPosition*, *PSize* and *USSize* flags for *XSetWMNormalHints*.

I've changed *_glfwPlatformSetWindowPos* and *_glfwPlatformSetWindowSize* to set the respective flags and also provide the position or the size respectively to the hint.

From my testing on Ubuntu, this should work fine. It even fixes the "windows" test in the *tests* folder.

Any thoughts?