Save and restore true-colour state with the cursor.

I spotted this myself while looking through the code in search of the
cause of the background-colour-erase bug: saving and restoring the
cursor via ESC 7 / ESC 8 ought to also save and restore the current
graphics rendition attributes including foreground and background
colour settings, but it was not saving and restoring the new
term->curr_truecolour along with term->curr_attr.

So there's now a term->save_truecolour to keep that in, and also a
term->alt_save_truecolour to take account of the fact that all the
saved cursor state variables get swapped out _again_ when switching
between the main and alternate screens.

(However, there is not a term->alt_truecolour to complete the cross
product, because the _active_ graphics rendition is carried over when
switching between the terminal screens; it's only the _saved_ one from
ESC 7 / ESC 8 that is saved separately. That's consistent with the
behaviour we've had all along for ordinary fg/bg colour selection.)

terminal.h

@@ -140,6 +140,7 @@ struct terminal_tag {
     /* ESC 7 saved state for the alternate screen */
     pos alt_savecurs;
     int alt_save_attr;
+    truecolour alt_save_truecolour;
     int alt_save_cset, alt_save_csattr;
     int alt_save_utf, alt_save_wnext;
     int alt_save_sco_acs;