Fix morph target prepass shader (#9013)

# Objective

Since 10f5c92, shadows were broken for models with morph target.

When #5703 was merged, the morph target code in `render/mesh.wgsl` was
correctly updated to use the new import syntax. However, similar code
exists in `prepass/prepass.wgsl`, but it was never update. (the reason
code is duplicated is that the `Vertex` struct is different for both
files).

## Solution

Update the code, so that shadows render correctly with morph targets.

Author: nicopap

crates/bevy_pbr/src/prepass/prepass.wgsl

@@ -57,18 +57,18 @@ struct VertexOutput {
 #ifdef MORPH_TARGETS
 fn morph_vertex(vertex_in: Vertex) -> Vertex {
     var vertex = vertex_in;
-    let weight_count = layer_count();
+    let weight_count = bevy_pbr::morph::layer_count();
     for (var i: u32 = 0u; i < weight_count; i ++) {
-        let weight = weight_at(i);
+        let weight = bevy_pbr::morph::weight_at(i);
         if weight == 0.0 {
             continue;
         }
-        vertex.position += weight * morph(vertex.index, position_offset, i);
+        vertex.position += weight * bevy_pbr::morph::morph(vertex.index, bevy_pbr::morph::position_offset, i);
 #ifdef VERTEX_NORMALS
-        vertex.normal += weight * morph(vertex.index, normal_offset, i);
+        vertex.normal += weight * bevy_pbr::morph::morph(vertex.index, bevy_pbr::morph::normal_offset, i);
 #endif
 #ifdef VERTEX_TANGENTS
-        vertex.tangent += vec4(weight * morph(vertex.index, tangent_offset, i), 0.0);
+        vertex.tangent += vec4(weight * bevy_pbr::morph::morph(vertex.index, bevy_pbr::morph::tangent_offset, i), 0.0);
 #endif
     }
     return vertex;