Add texture modifier rendering itemstacks

[Kindaglia]

• Goal of the PR
  Add a server-side Lua API to generate accurate inventory preview textures
  for any item, so mods like drawers can display correct 2D icons of
  complex nodes (stairs, slabs, nodeboxes, meshes) on world entities.

• How does the PR work?
  New Lua function core.get_item_inventory_texture(itemname) returns a
  virtual texture string [inventorypreview:<base64 itemstring>]. On the
  client, the texture source recognises this prefix, decodes the itemstring,
  and renders the item through the same drawItemStack pipeline used by
  the inventory GUI, producing an accurate 2D snapshot.

• Does it resolve any reported issue?
  Yes, fixes Nodes aren't correctly displayed in the drawer front. minetest-mods/drawers#60

• Does this relate to a goal in the roadmap?
  Not directly, but it improves the modding API and enables better visual
  fidelity for entity-based storage mods.

• If not a bug fix, why is this PR needed? What usecases does it solve?
  Without this API, mods can only approximate inventory icons using
  [inventorycube], which produces incorrect results for stairs, slabs,
  walls, and other non-fullblock nodes. This prevents drawer/barrel/crate
  mods from showing recognisable item previews on their entities.

• If you have used an LLM/AI to help with code or assets, you must disclose this.
  Yes -- this PR was created with assistance from opencode + GPT 5.5 xhigh

To do

This PR is Ready for Review.

How to test

.. code-block:: lua

local texture = core.get_item_inventory_texture("mcl_stairs:stair_oak")
local obj = minetest.add_entity(pos, "drawers:drawer")
obj:set_properties({
visual = "upright_sprite",
textures = { texture },
})

[Kindaglia]

drawer mod fixed here https://github.com/Kindaglia/drawers/tree/simplify-visual-rendering

[appgurueu]

Documentation is missing. No option to render to a desired resolution (64x64 is hardcoded) is also odd.

Add a server-side Lua API to generate accurate inventory preview textures for any item

To be precise, this is not what this is. This is effectively a texture modifier so clients can generate the proper texture (same as my PR #10570 back in the day, might want to compare), plus a convenience API wrapper to build this texture modifier.

[Kindaglia]

@appgurueu thanks for the review! Here's my plan:

1. Documentation — I'll add docs for both the [inventorypreview: texture modifier and core.get_item_inventory_texture() in doc/lua_api.md.

2. Resolution — I'll make the modifier support an optional :WxH suffix, e.g. [inventorypreview:<base64>:128x128, and add an optional size parameter (default 64) to core.get_item_inventory_texture(itemstring[, size]).

3. Comparison with your Item render-to-texture: Texture modifier, speedup #10570 — I chose base64-encoded itemstrings to avoid the : delimiter ambiguity you encountered and to support item metadata. Perhaps [inventorypreview:<base64>[:WxH] is reasonable syntax? Let me know what you think.