Make effect_id phase-local instead of global.

Make effect_id phase-local instead of global.

This is mainly to reduce the number of substantially identical shaders
we have to keep around and compile; even though two chains may be
different, often, some phase (and very often, a large one at that)
will be similar. However, in the old system, since effects had global
IDs, a change in an earlier phase would displace identifiers in a
later one, and the shader would be uncacheable.

Note that this means that an effect can actually have multiple effect_ids
now (since it could already be part of multiple phases). This is the
reason why we can't just keep having a single effect_id on the node
that we set phase-locally; it really needs to be different between phases.