The image node output is the default value when no image is selected. In pre-2.60 this was always initialized to 0 alpha, the defaults written in the node socket templates were completely ignored for outputs (except for value and RGB input nodes, which use these as button values). Now the stack values are initialized with the template defaults, which are all 0 by default. This patch changes alpha to 0 for image and render layer outputs too.
The value input nodes (like rgb input nodes) use the output value out their first socket for the button value. This needs explicitly set range limits to allow meaningful user input.
This would previously break because begin/end functions for each tree type still have some checks of the ntree->execdata pointer in them, despite the intended use of execdata instances instead of trees themselves for execution data storage. This is an artifact of the old execution system that required these checks to be made in the functions to avoid multiple execution of top-level trees. Now these functions take an additional argument, so group nodes can prevent them from setting and checking the nodetree->execdata pointers.
This branch adds mostly organizational improvements to the node system by renaming the node folders and files. A couple of internal features have been added too.
Detailed information can be found on the wiki page:
http://wiki.blender.org/index.php/User:Phonybone/Particles2010
