[Bf-cycles] Script node updating from OSL defaults

[Lukas Tönne]

> If it's possible to figure out programmatically what parameters are in their
> default settings and what have been modified, then upon updating a node type
> definition you could leave any modified parameters as-is and update
> parameters that were previously at the default, to the new default. Houdini
> acts like this, and it works pretty well.

Actually, for ID properties we already have is_property_set (or
whatever it was called in bpy): this will be false until the property
is first set by the user (and subsequently stored in ID properties).
However, this only works for ID properties and RNA runtime properties
respectively. In the customnodes branch i'm consistently using the
bpy.props system for such socket values, so it would work, but the
current nodes in trunk can not use it because they're DNA variables
...