Set the audio engine unmute button relative to its parent element

[docEdub]

The position of the audio engine unmute button in the playground is in the top left corner of the code pane instead of the render area.

This change fixes the issue by positioning the button relative to the parent element instead of absolute top left.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Snapshot stored with reference name:
refs/pull/16378/merge

Test environment:
https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/16378/merge/index.html

To test a playground add it to the URL, for example:

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/16378/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/16378/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/16378/merge
https://gui.babylonjs.com/?snapshot=refs/pull/16378/merge
https://nme.babylonjs.com/?snapshot=refs/pull/16378/merge

To test the snapshot in the playground with a playground ID add it after the snapshot query string:

https://playground.babylonjs.com/?snapshot=refs/pull/16378/merge#BCU1XR#0

[bjsplat]

Visualization tests for WebGPU

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/16378/merge/testResults/webgpuplaywright/index.html

[bjsplat]

WebGL2 visualization test reporter:

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/16378/merge/testResults/webgl2playwright/index.html