fix(postfx): drop duplicate VertexOutput in outline_shader (#48)

[proggeramlug]

Summary

Follow-up to #51. After that PR landed, @adevart re-tested and the bind-group validation error in `test-gltf-watch` was replaced by a shader compile error:

```
Shader 'outline_shader' parsing error: redefinition of `VertexOutput`
```

Both `FULLSCREEN_VERT` and `OUTLINE_FRAG` declared `struct VertexOutput`, which becomes a redefinition once `format!("{}\n{}", ...)` concatenates them in `PostFxPipeline::new`. This was a latent bug — naga was previously masking it behind the earlier bind-group filterable validation error that #51 fixed. Now that the bind-group error is gone, the shader-parse error surfaces and breaks the entire post-FX pipeline init on every example, which is why `sponza` started crashing on load and several others regressed.

Fix: remove the duplicate from `OUTLINE_FRAG`. The fragment shader picks up `VertexOutput` from the concatenated vertex shader prefix.

Test plan

• `cargo check` (`native/macos`)
• Validated the concatenated WGSL source with `naga 29.0.0` CLI — "Validation successful"
• Re-run `examples/test-gltf-watch` on macOS to confirm the redefinition error is gone and outlines render
• Re-run `examples/sponza` on macOS to confirm it no longer crashes on load (previously regressed by the shader compile failure)