Use a sorted Map for vertex buffer attributes (#1796)

The `VertexBufferLayout` returned by `crates\bevy_render\src\mesh\mesh.rs:308` was unstable, because `HashMap.iter()` has a random order. This caused the pipeline_compiler to wrongly consider a specialization to be different (`crates\bevy_render\src\pipeline\pipeline_compiler.rs:123`), causing each mesh changed event to potentially result in a different `PipelineSpecialization`. This in turn caused `Draw` to emit a `set_pipeline` much more often than needed.

This fix shaves off a `BindPipeline` and two `BindDescriptorSets` (for the Camera and for global renderresources) for every mesh after the first that can now use the same specialization, where it didn't before (which was random).

`StableHashMap` was not a good replacement, because it isn't `Clone`, so instead I replaced it with a `BTreeMap` which is OK in this instance, because there shouldn't be many insertions on `Mesh.attributes` after the mesh is created.

crates/bevy_render/src/mesh/mesh.rs

@@ -13,7 +13,7 @@ use bevy_ecs::{
 };
 use bevy_math::*;
 use bevy_reflect::TypeUuid;
-use std::borrow::Cow;
+use std::{borrow::Cow, collections::BTreeMap};
 
 use crate::pipeline::{InputStepMode, VertexAttribute, VertexBufferLayout};
 use bevy_utils::{HashMap, HashSet};
@@ -208,9 +208,11 @@ impl From<&Indices> for IndexFormat {
 #[uuid = "8ecbac0f-f545-4473-ad43-e1f4243af51e"]
 pub struct Mesh {
     primitive_topology: PrimitiveTopology,
-    /// `bevy_utils::HashMap` with all defined vertex attributes (Positions, Normals, ...) for this
+    /// `std::collections::BTreeMap` with all defined vertex attributes (Positions, Normals, ...) for this
     /// mesh. Attribute name maps to attribute values.
-    attributes: HashMap<Cow<'static, str>, VertexAttributeValues>,
+    /// Uses a BTreeMap because, unlike HashMap, it has a defined iteration order,
+    /// which allows easy stable VertexBuffers (i.e. same buffer order)
+    attributes: BTreeMap<Cow<'static, str>, VertexAttributeValues>,
     indices: Option<Indices>,
 }