libweston: Delete paint nodes when views change layers

Since we use paint nodes to track damage, we have to be very careful to
make sure that we never have paint nodes that aren't actually being
displayed on their outputs.

Shells may move views to invisible layers (minimized_layer, inactive_layer)
and this currently leaves the paint node alive but not visible on its
output.

When this happens, the paint node's previous visible region is left
undamaged, and the paint node is removed from processing.

Let's delete paint nodes when their views change layers, thus creating
damage for their full visible region.

This may create excess damage if the paint node moves between two visible
layers, but this is probably far less harmful than leaving pieces of
invisible views on screen.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>