Yet another attempt at fixing bugs #14602, 14630, 14669.

Yet another attempt at fixing bugs #14602, 14630, 14669.

 src/w32fns.c (w32_wnd_proc): Don't compute the header line and mode
 line dimensions here, to avoid race conditions with the main
 thread.
 src/w32term.c (w32_draw_window_cursor): Compute the header line and
 mode line dimensions here.
 <w32_system_caret_window, w32_system_caret_hdr_height>:
 <w32_system_caret_mode_height>: New variables.
 src/w32term.h: Declare them.