re-export cairo from piet-cairo (#505)

* re-export cairo from piet-cairo

This helps the user avoid errors from using
incompatible cairo versions.

* Update piet-cairo/src/lib.rs

Co-authored-by: Colin Rofls <[email protected]>

Author: euclio

piet-cairo/src/lib.rs

@@ -14,6 +14,8 @@ use piet::{
     LineJoin, RenderContext, StrokeStyle,
 };
 
+pub use cairo;
+
 pub use crate::text::{CairoText, CairoTextLayout, CairoTextLayoutBuilder};
 
 pub struct CairoRenderContext<'a> {