Split some textures into sprite sheets

- Split buttons.png into individual textures.
 - Split corners_xyz.png into the following:

   - borders_xyz.png: A nine-sliced texture of the computer borders.
   - pocket_bottom_xyz.png: A horizontally 3-sliced texture of the
     bottom part of a pocket computer.
   - sidebar_xyz.png: A vertically 3-sliced texture of the computer
     sidebar.

While not splitting the sliced textures into smaller ones may seem a
little odd, it's consistent with what vanilla does in 1.20.2, and I
think will make editing them easier than juggling 9 textures.

I do want to make this more data-driven in the future, but that will
have to wait until the changes in 1.20.2.

This also adds a tools/update-resources.py program, which performs this
transformation on a given resource pack.

tools/update-resources.py

@@ -0,0 +1,115 @@
+#!/usr/bin/env python3
+
+# SPDX-FileCopyrightText: 2023 The CC: Tweaked Developers
+#
+# SPDX-License-Identifier: MPL-2.0
+
+
+"""
+Upgrades textures to newer resource pack formats.
+
+Currently implemented transformations:
+ - Split gui/corners_*.png and gui/buttons.png textures into smaller textures.
+"""
+
+from PIL import Image
+import os
+import pathlib
+import argparse
+
+
+def box(x: int, y: int, w: int, h: int):
+    return (x, y, x + w, y + h)
+
+
+def unstitch_buttons(input_file: pathlib.Path):
+    """Unstitch the button texture,"""
+    buttons = Image.open(input_file)
+
+    output_dir = input_file.parent / "buttons"
+    output_dir.mkdir(exist_ok=True)
+
+    buttons.crop(box(1, 1, 12, 12)).save(output_dir / "turned_off.png")
+    buttons.crop(box(1, 15, 12, 12)).save(output_dir / "turned_off_hover.png")
+
+    buttons.crop(box(15, 1, 12, 12)).save(output_dir / "turned_on.png")
+    buttons.crop(box(15, 15, 12, 12)).save(output_dir / "turned_on_hover.png")
+
+    buttons.crop(box(29, 1, 12, 12)).save(output_dir / "terminate.png")
+    buttons.crop(box(29, 15, 12, 12)).save(output_dir / "terminate_hover.png")
+
+
+def unstitch_corners(input_file: pathlib.Path, family: str):
+    """Unstitch the corners texture."""
+
+    input = Image.open(input_file)
+    output_dir = input_file.parent
+
+    border = Image.new("RGBA", (36, 36))
+    # Corners
+    border.paste(input.crop(box(12, 28, 12, 12)), box(00, 00, 12, 12))
+    border.paste(input.crop(box(24, 28, 12, 12)), box(24, 00, 12, 12))
+    border.paste(input.crop(box(12, 40, 12, 12)), box(00, 24, 12, 12))
+    border.paste(input.crop(box(24, 40, 12, 12)), box(24, 24, 12, 12))
+    # Horizontal bars
+    border.paste(input.crop(box(00, 00, 12, 12)), box(12, 00, 12, 12))
+    border.paste(input.crop(box(00, 12, 12, 12)), box(12, 24, 12, 12))
+    # Vertical bars
+    border.paste(input.crop(box(00, 28, 12, 12)), box(00, 12, 12, 12))
+    border.paste(input.crop(box(36, 28, 12, 12)), box(24, 12, 12, 12))
+
+    border.save(output_dir / f"border_{family}.png")
+
+    if family != "command":
+        # Fatter bottom pocket computer border.
+        pocket_computer = Image.new("RGBA", (36, 20))
+        # Middle
+        pocket_computer.paste(input.crop(box(00, 56, 12, 20)), box(12, 0, 12, 20))
+        # Corners
+        pocket_computer.paste(input.crop(box(12, 80, 12, 20)), box(00, 0, 12, 20))
+        pocket_computer.paste(input.crop(box(24, 80, 12, 20)), box(24, 0, 12, 20))
+        pocket_computer.save(output_dir / f"pocket_bottom_{family}.png")
+
+    if family != "colour":
+        # Sidebar
+        sidebar = Image.new("RGBA", (17, 14))
+        sidebar.paste(input.crop(box(0, 102, 17, 14)), box(0, 0, 17, 14))
+        sidebar.save(output_dir / f"sidebar_{family}.png")
+
+
+def main() -> None:
+    spec = argparse.ArgumentParser()
+    spec.add_argument("dir", type=pathlib.Path)
+
+    dir: pathlib.Path = spec.parse_args().dir
+
+    texture_path = dir / "assets" / "computercraft" / "textures"
+
+    transformed: list[pathlib.Path] = []
+
+    buttons_path = texture_path / "gui" / "buttons.png"
+    if buttons_path.exists():
+        unstitch_buttons(buttons_path)
+        transformed.append(buttons_path)
+
+    for family in ("normal", "advanced", "command", "colour"):
+        path = texture_path / "gui" / f"corners_{family}.png"
+        if path.exists():
+            unstitch_corners(path, family)
+            transformed.append(path)
+
+    if len(transformed) == 0:
+        print("No files were transformed")
+        return
+
+    print("The following files may be deleted")
+    for file in transformed:
+        print(f" - {file}")
+
+    if input("Do so now? [y/N]").lower() == "y":
+        for file in transformed:
+            os.remove(file)
+
+
+if __name__ == "__main__":
+    main()