Currently there are a few callbacks on `bNodeType` that do the same
thing for every node type except reroutes and frame nodes. Having a
callback for basic things complicates code and makes it harder to
understand, and reroutes and frames are special cases in larger way.
Arguably frame nodes shouldn't even be drawn like regular nodes,
given that it adds a case of O(N^2) looping through all nodes.
"Unrolling" the callbacks makes it easier to see what's happening,
and therefore easier to optimize.
Differential Revision: https://developer.blender.org/D13463
338c1060d5