Fix meshlets when bindless disabled. (#17770)

# Objective

#16966 tried to fix a bug where
`slot` wasn't passed to `parallaxed_uv` when not running under bindless,
but failed to account for meshlets. This surfaces on macOS where
bindless is disabled.

## Solution

Lift the slot variable out of the bindless condition so it's always
available.

Author: tychedelia

crates/bevy_pbr/src/render/pbr_fragment.wgsl

@@ -70,17 +70,16 @@ fn pbr_input_from_standard_material(
     in: VertexOutput,
     is_front: bool,
 ) -> pbr_types::PbrInput {
-#ifdef BINDLESS
 #ifdef MESHLET_MESH_MATERIAL_PASS
     let slot = in.material_bind_group_slot;
 #else   // MESHLET_MESH_MATERIAL_PASS
     let slot = mesh[in.instance_index].material_and_lightmap_bind_group_slot & 0xffffu;
 #endif  // MESHLET_MESH_MATERIAL_PASS
+#ifdef BINDLESS
     let flags = pbr_bindings::material[slot].flags;
     let base_color = pbr_bindings::material[slot].base_color;
     let deferred_lighting_pass_id = pbr_bindings::material[slot].deferred_lighting_pass_id;
 #else   // BINDLESS
-    let slot = mesh[in.instance_index].material_and_lightmap_bind_group_slot & 0xffffu;
     let flags = pbr_bindings::material.flags;
     let base_color = pbr_bindings::material.base_color;
     let deferred_lighting_pass_id = pbr_bindings::material.deferred_lighting_pass_id;