Add Lua API function to resolve node/collision/selection boxes

[grorp]

For some purposes, it is useful for modders to be able to retrieve the actual collision or selection boxes of a node. This includes:

• painting on selection boxes (see my GGraffiti mod)
• predicting whether an entity will collide with anything before spawning it (see Make raycasts support collision boxes as well #12673)
• preventing players from placing nodes inside other players (see the Disable Build Where They Stand mod)

However, manually resolving node/collision/selection boxes in a node definition into actual boxes can be quite complex, depending on the type of box. Boxes can be facedir-rotated, wallmounted, leveled and/or connected. #11391 will further increase the number of cases to consider. There is a Lua implementation by @appgurueu: boxes.lua

This PR instead exposes the existing engine method for resolving boxes to the Lua API:

* `minetest.get_node_boxes(box_type, pos, [node])`
    * `box_type` must be `"node_box"`, `"collision_box"` or `"selection_box"`.
    * `pos` must be a node position.
    * `node` can be a table in the form `{name=string, param1=number, param2=number}`.
      If `node` is `nil`, the actual node at `pos` is used instead.
    * Resolves any facedir-rotated boxes, connected boxes and the like into
      actual boxes.
    * Returns a list of boxes in the form
      `{{x1, y1, z1, x2, y2, z2}, {x1, y1, z1, x2, y2, z2}, ...}`. Coordinates
      are relative to `pos`.

I also added the "Node Box Visualizer", a Devtest tool for testing the new function.

To do

This PR is a Ready for Review.

How to test

Start a Devtest world and get yourself the "Node Box Visualizer" tool. Press the place button to change the selected box type. Punch a node to show its boxes of the selected type.

[sfan5]

Idea: add a variant of the api that can work with a {name=...,param1=....,param2=...} table
This answers the question "what would the boxes of this node look like" (with the caveat that connected ones wouldn't work).

[grorp]

Idea: add a variant of the api that can work with a {name=...,param1=....,param2=...} table This answers the question "what would the boxes of this node look like" (with the caveat that connected ones wouldn't work).

That's a good idea, I've implemented it.

[appgurueu]

Two comments are out of date, otherwise looks good, tested and works.

Also a nitpick / detail: Maybe it would make sense to return nil for collision boxes if a node is not walkable / nil for selection boxes if a node is not pointable / nil for node boxes if a node doesn't use node boxes.

[grorp]

Also a nitpick / detail: Maybe it would make sense to return nil for collision boxes if a node is not walkable / nil for selection boxes if a node is not pointable / nil for node boxes if a node doesn't use node boxes.

For selection boxes, returning nil if pointable = false doesn't make sense anymore since #13992 was merged. Conceptually, you could argue that the boxes are still there, they're just not used.