Don't send compositor's global key bindings to the input method

Although weston_compositor_run_key_binding() is called when the current
keyboard grab is default_grab or input_method_grab, swallowing the key
event is processed only on default_grab. As a result key events that
should be swallowed are sent to the input method unexpectedly.

For example, when a user press `Super + s` on weston-editor to take a
screen shot, `s` will be unexpectedly entered to the text area.
I confirmed such behaviour with weston-simple-im and fcitx5-5.0.10.
It doesn't occur with weston-keyboard because it doesn't install
keyboard grab.

Signed-off-by: Takuro Ashie <ashie@clear-code.com>