Break phases when a node needs both to supply mipmaps and _not_ supply mipmaps.

Break phases when a node needs both to supply mipmaps and _not_ supply mipmaps.

In most cases, this means that we can keep a local copy that supplies
mipmaps and an RTT input that doesn't, or the other way around.

This is not a complete solution, since it doesn't take into account
the case where _both_ inputs will be RTT, but it's good enough for now.

Also requires us to distinguish between links between nodes in the same phase
and in different phases, or we'd get GLSL compile errors as we had both the
local and the RTT input being called e.g. “in0”.