Split up turtle textures (#1813)

Turtles currently read their textures from a single 128x128 sprite
sheet. Most of this texture is unused which means we end up wasting a
lot of the block texture atlas[^1].

This change splits up the turtle textures into individual 32x32
textures[^2], one for each side, and then an additional backpack
texture.

I'm very sorry to any resource pack artists out there. The
tools/update-resources.py script will update existing packs, but does
not (currently) handle non-standard resolutions.

[^1]: It used to be worse: https://github.com/dan200/ComputerCraft/issues/145

[^2]: Turtle textures are a bit weird, in that they mostly *look* 16x16,
  but have some detail in places.

buildSrc/src/main/kotlin/cc/tweaked/gradle/MergeTrees.kt

@@ -98,7 +98,6 @@ abstract class MergeTrees : DefaultTask() {
         }
 
         val sharedFiles = files.entries.asSequence().filter { (_, v) -> v.found == sources.size }.map { (k, _) -> k }.toList()
-        println(sharedFiles)
 
         // Copy shared files to the common directory
         fsOperations.sync {