Compute vertex_count for indexed meshes on GpuMesh (#8460)

# Objective

Compute the `vertex_count` for indexed meshes as well as non-indexed
meshes.

I will need this in a future PR based on #8427 that adds a gizmo
component that draws the normals of a mesh when attached to an entity
([branch](https://github.com/devil-ira/bevy/compare/instanced-line-rendering...devil-ira:bevy:instanced-line-rendering-normals)).

<details><summary>Example image</summary>
<p>


![image](https://user-images.githubusercontent.com/29694403/233789526-cb5feb47-0aa7-4e69-90a2-e31ec24aadff.png)

</p>
</details> 

## Solution

Move `vertex_count` field from `GpuBufferInfo::NonIndexed` to `GpuMesh`

## Migration Guide

`vertex_count` is now stored directly on `GpuMesh` instead of
`GpuBufferInfo::NonIndexed`.

crates/bevy_pbr/src/render/mesh.rs

@@ -1177,8 +1177,8 @@ impl<P: PhaseItem> RenderCommand<P> for DrawMesh {
                     pass.set_index_buffer(buffer.slice(..), 0, *index_format);
                     pass.draw_indexed(0..*count, 0, 0..1);
                 }
-                GpuBufferInfo::NonIndexed { vertex_count } => {
-                    pass.draw(0..*vertex_count, 0..1);
+                GpuBufferInfo::NonIndexed => {
+                    pass.draw(0..gpu_mesh.vertex_count, 0..1);
                 }
             }
             RenderCommandResult::Success

crates/bevy_render/src/mesh/mesh/mod.rs

@@ -820,6 +820,7 @@ impl From<&Indices> for IndexFormat {
 pub struct GpuMesh {
     /// Contains all attribute data for each vertex.
     pub vertex_buffer: Buffer,
+    pub vertex_count: u32,
     pub buffer_info: GpuBufferInfo,
     pub primitive_topology: PrimitiveTopology,
     pub layout: MeshVertexBufferLayout,
@@ -834,9 +835,7 @@ pub enum GpuBufferInfo {
         count: u32,
         index_format: IndexFormat,
     },
-    NonIndexed {
-        vertex_count: u32,
-    },
+    NonIndexed,
 }
 
 impl RenderAsset for Mesh {
@@ -861,11 +860,8 @@ impl RenderAsset for Mesh {
             contents: &vertex_buffer_data,
         });
 
-        let buffer_info = mesh.get_index_buffer_bytes().map_or(
-            GpuBufferInfo::NonIndexed {
-                vertex_count: mesh.count_vertices() as u32,
-            },
-            |data| GpuBufferInfo::Indexed {
+        let buffer_info = if let Some(data) = mesh.get_index_buffer_bytes() {
+            GpuBufferInfo::Indexed {
                 buffer: render_device.create_buffer_with_data(&BufferInitDescriptor {
                     usage: BufferUsages::INDEX,
                     contents: data,
@@ -873,13 +869,16 @@ impl RenderAsset for Mesh {
                 }),
                 count: mesh.indices().unwrap().len() as u32,
                 index_format: mesh.indices().unwrap().into(),
-            },
-        );
+            }
+        } else {
+            GpuBufferInfo::NonIndexed
+        };
 
         let mesh_vertex_buffer_layout = mesh.get_mesh_vertex_buffer_layout();
 
         Ok(GpuMesh {
             vertex_buffer,
+            vertex_count: mesh.count_vertices() as u32,
             buffer_info,
             primitive_topology: mesh.primitive_topology(),
             layout: mesh_vertex_buffer_layout,

crates/bevy_sprite/src/mesh2d/mesh.rs

@@ -599,8 +599,8 @@ impl<P: PhaseItem> RenderCommand<P> for DrawMesh2d {
                     pass.set_index_buffer(buffer.slice(..), 0, *index_format);
                     pass.draw_indexed(0..*count, 0, 0..1);
                 }
-                GpuBufferInfo::NonIndexed { vertex_count } => {
-                    pass.draw(0..*vertex_count, 0..1);
+                GpuBufferInfo::NonIndexed => {
+                    pass.draw(0..gpu_mesh.vertex_count, 0..1);
                 }
             }
             RenderCommandResult::Success

examples/shader/shader_instancing.rs

@@ -253,8 +253,8 @@ impl<P: PhaseItem> RenderCommand<P> for DrawMeshInstanced {
                 pass.set_index_buffer(buffer.slice(..), 0, *index_format);
                 pass.draw_indexed(0..*count, 0, 0..instance_buffer.length as u32);
             }
-            GpuBufferInfo::NonIndexed { vertex_count } => {
-                pass.draw(0..*vertex_count, 0..instance_buffer.length as u32);
+            GpuBufferInfo::NonIndexed => {
+                pass.draw(0..gpu_mesh.vertex_count, 0..instance_buffer.length as u32);
             }
         }
         RenderCommandResult::Success