Drop unnecessary NULL check in frame_callback_handle_done()

After wl_callback_destroy() is called, frame_callback_handle_done()
is guaranteed to never be called by libwayland: the done event will
be silently dropped instead.

Fix wl_callback memory leak

And better behave when multiple configure events are received.

Fix build error due to decl after case

Fixes the following error:

    ../main.c:267:4: error: expected expression
                            char *end = NULL;
                            ^

Fix return value after input event

Add -d option to specify delay

readme: add links to ML and IRC channel

Initial commit