Make `no_std` unconditional in `vello_cpu`

sparse_strips/vello_cpu/src/dispatch/multi_threaded.rs

diff --git a/sparse_strips/vello_cpu/src/dispatch/multi_threaded.rs b/sparse_strips/vello_cpu/src/dispatch/multi_threaded.rs
index 6cb3fe1..623dc70 100644
--- a/sparse_strips/vello_cpu/src/dispatch/multi_threaded.rs
+++ b/sparse_strips/vello_cpu/src/dispatch/multi_threaded.rs
@@ -59,7 +59,7 @@
             .unwrap();
         let alpha_storage = Arc::new(OnceLockAlphaStorage::new(num_threads));
         let workers = Arc::new(ThreadLocal::new());
-        let task_batch = vec![];
+        let task_batch = std::vec![];
 
         {
             let alpha_storage = alpha_storage.clone();

sparse_strips/vello_cpu/src/lib.rs

diff --git a/sparse_strips/vello_cpu/src/lib.rs b/sparse_strips/vello_cpu/src/lib.rs
index 0e0c412..9ae26f3 100644
--- a/sparse_strips/vello_cpu/src/lib.rs
+++ b/sparse_strips/vello_cpu/src/lib.rs
@@ -103,10 +103,12 @@
     clippy::cast_possible_truncation,
     reason = "We cast u16s to u8 in various places where we know for sure that it's < 256"
 )]
-#![cfg_attr(not(feature = "multithreading"), no_std)]
+#![no_std]
 
 extern crate alloc;
 extern crate core;
+#[cfg(feature = "std")]
+extern crate std;
 
 mod render;