fix msaa shift with irradiance volumes in mesh pipeline key (#12845)

# Objective

- #12791 broke example `irradiance_volumes`
- Fixes #12876 

```
wgpu error: Validation Error

Caused by:
    In Device::create_render_pipeline
      note: label = `pbr_opaque_mesh_pipeline`
    Color state [0] is invalid
    Sample count 8 is not supported by format Rgba8UnormSrgb on this device. The WebGPU spec guarentees [1, 4] samples are supported by this format. With the TEXTURE_ADAPTER_SPECIFIC_FORMAT_FEATURES feature your device supports [1, 2, 4].
```

## Solution

- Shift bits a bit more

crates/bevy_pbr/src/render/mesh.rs

@@ -576,7 +576,7 @@ bitflags::bitflags! {
 
 impl MeshPipelineKey {
     const MSAA_MASK_BITS: u32 = 0b111;
-    const MSAA_SHIFT_BITS: u32 = Self::LAST_FLAG.bits().trailing_zeros();
+    const MSAA_SHIFT_BITS: u32 = Self::LAST_FLAG.bits().trailing_zeros() + 1;
 
     const BLEND_MASK_BITS: u32 = 0b11;
     const BLEND_SHIFT_BITS: u32 = Self::MSAA_MASK_BITS.count_ones() + Self::MSAA_SHIFT_BITS;