dix: do not send focus event when grab actually does not change
c67f2eac ("dix: always send focus event on grab change") made dix always sent events when it's a NotifyGrab or NotifyUngrab, even if from == to, because 'from' can just come from a previous XSetInputFocus call.
However, when an application calls XGrabKeyboard several times on the same window, we are now sending spurious FocusOut+FocusIn with NotifyGrab, even if the grab does not actually change. This makes screen readers for blind people spuriously emit activity events which disturb screen reading workflow when e.g. switching between menus.
This commit avoids calling DoFocusEvents in that precise case, i.e. when oldWin is a previous grab and the new grab is the same window.