Fix dropdown interaction

[Byakkl]

This fix enables the PerformRaycast method to instead go through all TrackedDeviceRaycasters in order to get the correct element while respecting canvas sorting order.
The current method only returns the first hit from the first TrackedDeviceRaycaster preventing nested canvas components from returning the correct element being selected which causes dropdown menus to be unusable as they have their own canvas and TrackedDeviceRaycaster.

Checks to compare canvas sorting order, ensures that the closest element chosen is correct in the case of overlapping canvases. This would occur if a dropdown menu covers another UI element. GetComponentInParent on the raycast gameobject is required due to protection level of the canvas variable in TrackedDeviceRaycaster

This resolves issue #1087

[unity-cla-assistant]

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1 out of 2 committers have signed the CLA.

✅ ekcoh
❌ Travis Milne

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[ekcoh]

Updated branch with critical CI fix from develop. Hope it doesn't cause inconvenience.