Improve documentation about how to write a painter for egui

epaint/src/triangles.rs

@@ -24,6 +24,8 @@ pub struct Vertex {
 #[derive(Clone, Debug, Default)]
 pub struct Triangles {
     /// Draw as triangles (i.e. the length is always multiple of three).
+    ///
+    /// Egui is NOT consistent with what winding order it uses, so turn off backface culling.
     pub indices: Vec<u32>,
 
     /// The vertex data indexed by `indices`.