Correctly register focus events concomitant with alpha changes

Correctly register focus events concomitant with alpha changes

* src/xterm.c (x_frame_highlight, x_frame_unhighlight): Skip
changing the frame alpha when the frame is not eligible for
focus state-specific alpha values; otherwise, the alpha might be
reset by the time a alpha change wrought by a focus change
arrives, impeding handle_one_xevent from subsequently restoring
the initial value.  (bug#66398)