Blender Git Loki

Cleanup: Consistent names for draw callbacks in bNodeType.

This aims to establish a common pattern for the various confusing draw callback function pointers in bNodeType:

draw_<purpose>_<nodetype>[_ex]

Currently there are 4 different types of draw callbacks:
* draw_nodetype, draw_nodetype_prepare: Main draw functions, allows specialized node drawing for things like frames and reroute nodes. Not exposed in the API.
* draw_buttons, draw_buttons_ex: Optional non-socket buttons, most commonly used callback. Extended version used in sidebar for verbose buttons that don't fit into a node.
* draw_backdrop: Draw elements in the backdrop (compositor only). Not exposed in the API.
* draw_input, draw_output: Specialized socket drawing for some nodes, only for OutputFile node. Should not be used any further and be removed at some point. Not exposed in the API.