window: don't call output configure uninitialised

window: don't call output configure uninitialised

Callbacks registered via display_set_output_configure_handler() are
promised to be called when we know the current mode for the output. If
the following order of events happens:
1. toytoolkit binds to a wl_output global
2. application registers an output configure handler
3. the wl_output.mode events are received

Then in step 2 we would call the callback with uninitialised output
informations, giving it a 0x0 size.

To avoid such race, do not call the callback from
display_set_output_configure_handler() if the output has 0x0 size.

The wl_output.mode event will be received later, and that will trigger
the right call to the callback.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>