MeshAllocator: Take padding elements into account in the MeshBufferSlice's range

[beicause]

Objective

When using Indices::U16 (or custom vertex attributes that have padding elements), the MeshAllocator::mesh_index_slice doesn't take padding elems into account.
For example, run the lines example with the following change:

diff --git a/examples/3d/lines.rs b/examples/3d/lines.rs
index c29e9fb45..c9fcff054 100644
--- a/examples/3d/lines.rs
+++ b/examples/3d/lines.rs
@@ -121,6 +121,13 @@ impl From<LineStrip> for Mesh {
             PrimitiveTopology::LineStrip,
             RenderAssetUsages::RENDER_WORLD,
         )
+        .with_inserted_indices(bevy::mesh::Indices::U16(
+            line.points
+                .iter()
+                .enumerate()
+                .map(|(i, _v)| i as u16)
+                .collect(),
+        ))
         // Add the point positions as an attribute
         .with_inserted_attribute(Mesh::ATTRIBUTE_POSITION, line.points)
     }

The index count of LineStrip should be 3 with 1 padding in the slab, which should be 3 vertexes but RenderDoc shows 4 vertexes:

Solution

Fix MeshAllocator by taking padding elements into account and subtract it from the end of the range.

Testing

Run lines example with above patch, the index count is correct: