window-x11: Consistently include the bottom border on shaded windows

We already do it in the theme code, but not the actual WM code. Since
we include the left/right borders, it only seems fair to include the
bottom border.

This effectively makes it so that shading a window means that the
client window "slot" has 0 height.
window-x11: Don't bother calculating the actual deltas for the frame

It's not necessary. All we care about is whether the size / position
has changed.
window-x11: Simplify leftovers from the static gravity code removal
window-x11: Calculate size_dx / size_dy before assigning to window->rect

This makes sure that we grow / shrink the frame and window in the
correct order.
window-x11: Rearrange locals a tiny bit