backend-wayland: Don't signal out resize events for activation

This should avoid spurious signal events received by the shell in order
to resize its buffer.

As we can have resize events being sent even if xdg-shell activation
doesn't happen, make use of the output dimensions to determine if we
need do send out a resize event or not.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>