Keep unsafe code forbidden when puffin is disabled (#3603)

This helps document what unsafe is being used for, and prevent other
uses from going unnoticed.

crates/epaint/src/lib.rs

@@ -22,7 +22,8 @@
 
 #![allow(clippy::float_cmp)]
 #![allow(clippy::manual_range_contains)]
-#![deny(unsafe_code)]
+#![cfg_attr(feature = "puffin", deny(unsafe_code))]
+#![cfg_attr(not(feature = "puffin"), forbid(unsafe_code))]
 
 mod bezier;
 pub mod image;