Don't make `std` prelude available in `vello_cpu` (#1097)

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..dfa97e9 100644
--- a/sparse_strips/vello_cpu/src/dispatch/multi_threaded.rs
+++ b/sparse_strips/vello_cpu/src/dispatch/multi_threaded.rs
@@ -10,6 +10,7 @@
 use crate::strip_generator::StripGenerator;
 use alloc::boxed::Box;
 use alloc::sync::Arc;
+use alloc::vec;
 use alloc::vec::Vec;
 use core::fmt::{Debug, Formatter};
 use crossbeam_channel::TryRecvError;

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;