Parallell tessellation (#3934)

* Part of https://github.com/emilk/egui/issues/1485

This adds a `rayon` feature to `epaint` and `egui` to parallelize
tessellation of large shapes, such as high-resolution plot lines.

crates/epaint/src/lib.rs

@@ -52,13 +52,16 @@ pub use {
     },
     stats::PaintStats,
     stroke::Stroke,
-    tessellator::{tessellate_shapes, TessellationOptions, Tessellator},
+    tessellator::{TessellationOptions, Tessellator},
     text::{FontFamily, FontId, Fonts, Galley},
     texture_atlas::TextureAtlas,
     texture_handle::TextureHandle,
     textures::TextureManager,
 };
 
+#[allow(deprecated)]
+pub use tessellator::tessellate_shapes;
+
 pub use ecolor::{Color32, Hsva, HsvaGamma, Rgba};
 pub use emath::{pos2, vec2, Pos2, Rect, Vec2};
 
@@ -172,3 +175,38 @@ pub(crate) fn f64_hash<H: std::hash::Hasher>(state: &mut H, f: f64) {
         f.to_bits().hash(state);
     }
 }
+
+// ---------------------------------------------------------------------------
+
+/// Was epaint compiled with the `rayon` feature?
+pub const HAS_RAYON: bool = cfg!(feature = "rayon");
+
+// ---------------------------------------------------------------------------
+
+mod profiling_scopes {
+    #![allow(unused_macros)]
+    #![allow(unused_imports)]
+
+    /// Profiling macro for feature "puffin"
+    macro_rules! profile_function {
+        ($($arg: tt)*) => {
+            #[cfg(feature = "puffin")]
+            #[cfg(not(target_arch = "wasm32"))] // Disabled on web because of the coarse 1ms clock resolution there.
+            puffin::profile_function!($($arg)*);
+        };
+    }
+    pub(crate) use profile_function;
+
+    /// Profiling macro for feature "puffin"
+    macro_rules! profile_scope {
+        ($($arg: tt)*) => {
+            #[cfg(feature = "puffin")]
+            #[cfg(not(target_arch = "wasm32"))] // Disabled on web because of the coarse 1ms clock resolution there.
+            puffin::profile_scope!($($arg)*);
+        };
+    }
+    pub(crate) use profile_scope;
+}
+
+#[allow(unused_imports)]
+pub(crate) use profiling_scopes::*;