mods: gate love.thread behind a compute permission

Threads open a fresh Lua state with a full standard library, so they
stayed blocked wholesale. PotatoVoxel's prebuilder wants to run its
pure geometry phase on worker threads; the mod declares the new
"compute" permission and the sandbox hands out love.thread only
then. The worker runs the mod's own source (source-only, like every
mod file) and receives data only through channels.

src/mods/Manifest.lua

@@ -12,7 +12,7 @@ local Manifest = {}
 Manifest.PROFILES = { content = true, overhaul = true, total_conversion = true }
 Manifest.PERMISSIONS = { network = true, filesystem = true,
                          engine_internals = true, steps = true,
-                         background = true }
+                         background = true, compute = true }
 
 -- link-relevant registries; a mod that writes into one of these while
 -- declaring affects_link = false gets an attributed warning from the loader