Screen shaders don't combine
Only the last screen shader in the tree will be
used if multiple screen shaders are in the tree. In
order to have multiple screen shaders overlaid on
top of each other, a CanvasLayer needs
to be used.
For each screen shader, put it under a parent of
type CanvasLayer, and set the
layer property of the
CanvasLayer to the correct number such
that the effects are in the desired order.