WGSL: use correct syntax for matrix access (#5039)

# Objective

- `.x` is not the correct syntax to access a column in a matrix in WGSL: https://www.w3.org/TR/WGSL/#matrix-access-expr
- naga accepts it and translates it correctly, but it's not valid when shaders are kept as is and used directly in WGSL

## Solution

- Use the correct syntax

crates/bevy_pbr/src/render/skinning.wgsl

@@ -11,10 +11,10 @@ fn add_matrix(
     b: mat4x4<f32>,
 ) -> mat4x4<f32> {
     return mat4x4<f32>(
-        a.x + b.x,
+        a[0] + b[0],
-        a.y + b.y,
+        a[1] + b[1],
-        a.z + b.z,
+        a[2] + b[2],
-        a.w + b.w,
+        a[3] + b[3],
     );
 }
 
@@ -29,10 +29,10 @@ fn skin_model(
 }
 
 fn inverse_transpose_3x3(in: mat3x3<f32>) -> mat3x3<f32> {
-    let x = cross(in.y, in.z);
+    let x = cross(in[1], in[2]);
-    let y = cross(in.z, in.x);
+    let y = cross(in[2], in[0]);
-    let z = cross(in.x, in.y);
+    let z = cross(in[0], in[1]);
-    let det = dot(in.z, z);
+    let det = dot(in[2], z);
     return mat3x3<f32>(
         x / det,
         y / det,