Blender Git Loki

Cleanup: Store runtime space node variables in a separate struct

This commit moves runtime-only variables from the `SpaceNode`
DNA struct to a private struct in `node_intern.h`. Before, it was hard
to tell which data needed to be saved in files, this should make it
more clear.

Node that the `edittree` field is basically a runtime variable, since
it's set from the `treepath` list on read, but moving it would require
some more invasive changes that I don't think are worth it right now.
Also, not all of the moved variables were explicitly cleared on read--
`aspect` is set at the start of a redraw, `cursor` is set in a region
callback, and `recalc` was used as an update flag.

Differential Revision: https://developer.blender.org/D10141