clutter: Enable negative offsets in delete surrounding text

The input method can assign a negative value to
clutter_input_method_delete_surrounding() to move the cursor to the left.
But Wayland protocol accepts positive values in delete_surrounding() and
GTK converts the values to the negative ones in
text_input_delete_surrounding_text_apply().

GNOME/mutter#539
5 jobs for !517 with ibus-delete-surrounding in 5 minutes and 20 seconds
latest detached
Status Job ID Name Coverage
  Review
passed #393035
check-commit-log

00:00:25

 
  Build
passed #393036
build-mutter

00:01:42

passed #393037
build-without-native-backend

00:01:50

 
  Test
passed #393039
can-build-gnome-shell

00:01:30

passed #393038
test-mutter

00:03:03