Make `multithreading` require `std`

sparse_strips/vello_common/Cargo.toml

diff --git a/sparse_strips/vello_common/Cargo.toml b/sparse_strips/vello_common/Cargo.toml
index 08bf3dc..caf7121 100644
--- a/sparse_strips/vello_common/Cargo.toml
+++ b/sparse_strips/vello_common/Cargo.toml
@@ -37,7 +37,7 @@
 # Allow loading Pixmap from PNG, and drawing png glyphs.
 png = ["std", "dep:png"]
 # Enable multi-threaded rendering.
-multithreading = []
+multithreading = ["std"]
 # Add support for text rendering
 text = ["dep:skrifa"]
 

sparse_strips/vello_common/src/lib.rs

diff --git a/sparse_strips/vello_common/src/lib.rs b/sparse_strips/vello_common/src/lib.rs
index e7e2b0e..5998dde 100644
--- a/sparse_strips/vello_common/src/lib.rs
+++ b/sparse_strips/vello_common/src/lib.rs
@@ -51,13 +51,15 @@
     reason = "We temporarily ignore those because the casts\
 only break in edge cases, and some of them are also only related to conversions from f64 to f32."
 )]
-#![cfg_attr(not(feature = "multithreading"), no_std)]
+#![no_std]
 
 // Suppress the unused_crate_dependencies lint when both std and libm are specified.
 #[cfg(all(feature = "std", feature = "libm"))]
 use libm as _;
 
 extern crate alloc;
+#[cfg(feature = "std")]
+extern crate std;
 
 pub mod blurred_rounded_rect;
 pub mod coarse;