Getting eye parameters requires memory allocation

[asajeffrey]

One of the webvr issues in Servo is that we freeze the eye parameters for a VRDisplay when the object is created, so if they change then the changes don't get seen by JS. servo/servo#22758

The obvious fix for this is to get the eye parameters from rust-webvr, but unfortunately the API for that is to get the VRDisplayData

rust-webvr/rust-webvr-api/src/vr_display.rs

Lines 12 to 13 in 4e30999

	/// Returns the current display data.
	fn data(&self) -> VRDisplayData;

and the display data includes the display_name as a String

rust-webvr/rust-webvr-api/src/vr_display_data.rs

Lines 7 to 15 in 4e30999

	pub struct VRDisplayData {
	pub display_id: u32,
	pub display_name: String,
	pub connected: bool,
	pub capabilities: VRDisplayCapabilities,
	pub stage_parameters: Option<VRStageParameters>,
	pub left_eye_parameters: VREyeParameters,
	pub right_eye_parameters: VREyeParameters,
	}

Since the name is a String, it has to be freshly allocated each time the display data is accessed, which is wasteful.

Possible fixes: separate out the eye parameters from the display data, or make the display_name a &'static str or Cow<'static, str> rather than a String.