gsk: Add high depth rendernode api

gsk: Add high depth rendernode api

Add private api to find out if the content
of a render node should be considered 'deep'.

The information is collected at creation time,
so there is no tree-walking involved when we
are using this information in the renderer.

Currently, this comes down to whether there are
any texture nodes with high depth textures in the subtree.
In the future, we may want to allow marking gradient
nodes in this way as well.