ui/vc: skip text console resize when possible

This function is called on invalidate, on each cursor blink.

Avoid the extra copy when the console size didn't change.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230830093843.3531473-41-marcandre.lureau@redhat.com>