(w->base_line_number): Rework the way we flush the cache

(w->base_line_number): Rework the way we flush the cache

* src/xdisp.c (BASE_LINE_NUMBER_VALID_P): New macro.
(try_scrolling): Use it.
(redisplay_window, Fformat_mode_line): Use it to flush the
base_line_number (if it's stale) once at the beginning.
(decode_mode_spec): Don't use (or set) `w->start` and
`w->base_line_number` when operating on another buffer!