prevent resizing of fixed-size window

This is a pretty trivial fix for preventing resizing of windows on the server-side, if they have indicated a fixed-size constraint.

From the commit:

A client defines a fixed window size by setting identical minimum and
maximum size. While this is correctly detected by mutter and recorded by
setting flag 'has_resize_func' to false, this is previously not checked
when a client attempts to interactively resize the window.
This is fixed by checking the 'has_resize_func' flag and ignoring the
resize request if it is not set.

mutter already sets has_resize_func correctly if the minimum and maximum toplevel sizes are identical, but this seems to be ignored on Wayland when xdg_toplevel_resize is evoked by the client.

This patch fixes this by simply returning from xdg_toplevel_resize if has_resize_func is set and thus skipping all the resize logic. This is the same behaviour as implemented for xdg_toplevel_set_maximized in 6fe46cac.

Fixes #1331 (closed).