Fix for Tile image node. This was still using the path string for multilayer output instead of the dedicated layer name string.

When generating enums for the image node render layers, replace empty strings with a simple space character. This little hack allows the enum item to be selected from the dropdown list.

Partial revert of r46542.
Displaying image layers with flattened names (render_layer.render_pass) can lead to extremely long socket lists, especially with older files that were used with distinct render layers before. In general the flattening of names would be preferable, but it needs a sensible way of limiting the number of displayed outputs. For now the render layer dropdown selection is the best option to ensure usability.

Fix #31369 (discussed in bug report comments). Setting the path/layer name strings of the file/layer slots of the file output node would crash, due to missing id.data pointer in the PointerRNA. Solved by using the iter->parent.id.data pointer for the RNA collection iterator.

Fix #31472: displace node wrong y derivative (simple typo). Credit to Ervin Weber (lusque) for the fix.

A generalization of the modal node linking operator (for dragging from socket to socket).

This operator still had some built-in assumptions about the connectivity of input/output sockets (1-to-n in all current node systems). For future node systems (e.g. flow-based particles) and for general customizable nodes the operator is now fully symmetric and supports all kinds of connectivity limits (1:1, 1:n, m:1, m:n).

The operator data can also store a list of node links as opposed to a single link now, so that multiple links can be redirected at once. Holding the CTRL key when clicking a socket, all links from/to that socket are detached and can be moved to a different socket. This is useful for quickly appending a node without moving every individual link.

Fix for UI layout alignment of fixed-size items (including those with width < min. width). Such layout items were ignoring the center and right alignment in layouts before.

A couple more changes to the file and image nodes to improve access to layers that don't follow Blender's rlayer.rpass naming scheme.

--- Changes to File Output node ---
* Flat layer names in EXR multilayer files.

For a socket with name "AAA" the previous resulting EXR layer name would be "AAA.AAA", i.e. the render layer as well as render pass would use the socket name.

Now the "render_layer.render_pass" scheme is ignored in multilayer files, socket names are directly written to EXR layers (EXR layer name is "AAA" in this example). If sockets should have a notion of "render layer" this can still be achieved by explicitly adding a separator, e.g. "AAA.BBB". When loading such layers into a Blender Image struct, the name is interpreted as a "render_layer.render_pass" again (although the image node does not care about it, see below).

* Socket sub-paths (for singlelayer) or layer names (for multilayer) are stored in dedicated string variables in the socket storage data. This way the RNA can define precise string subtypes (PROP_FILEPATH) and length. The file/layer slots are defined as separate structs with own name properties in the RNA as well, so they can be used nicely with the list template.

* Ensure unique socket paths/layer names to prevent overwriting of files and layers respectively.

--- Changes to Image node ---
* Loading multilayer OpenEXR files has improved layer name splitting into render layer + render pass names now. This properly supports arbitrary EXR layer names now.
Example:

OpenEXR layer name: AAA.BBB.CCC

is split into

Render layer name: AAA.BBB
Render pass name: CCC

If the layer name has no '.' separators the render layer name is empty.

* Image node ignores the selected render layer in the image user data. Instead all existing layers are displayed at the same time by combining the render layer names with render pass names again, to reconstruct the original EXR layer name. This avoids the problem that render layers with empty name are not selectetable in the dropdown and allows using all image layers at the same time without duplicating the node.

Fix #31369. Needed some do_versions backward compatibility code to ensure the old file path was split correctly into base path + filename.

Fix #31350, by Sergey Sharybin.

This happens because of how output node index is initializing in assign_index function: itterator goes to the beginning of the nodes list using node->prev and then reviews the whole node list to find first unused index. The problem is that node's initialization now is getting called before node was added to node tree, so all output nodes have got equal index.