Fix a few integer-overflow glitches

Fix a few integer-overflow glitches

* src/composite.c (composition_compute_stop_pos, composition_reseat_it):
* src/dispextern.h (struct composition_it.rule_idx):
* src/keyboard.c (Fset__this_command_keys):
* src/xwidget.c (webkit_js_to_lisp):
Don’t assume object sizes fit in ‘int’.
* src/xwidget.c (Fxwidget_resize):
Don’t assume Emacs integers fit in ‘int’.