Move swapchain image enumeration into openxr initialization

[jdm]

Calling enumerate_images on the swapchains every frame is unnecessary:

webxr/webxr/openxr/mod.rs

Lines 501 to 505 in 824e416

	// TODO: the enumeration only needs to happen at initialization, not every frame.
	let left_swapchain_images = self.left_swapchain.enumerate_images().unwrap();
	let left_image = left_swapchain_images[self.left_image as usize];
	let right_swapchain_images = self.right_swapchain.enumerate_images().unwrap();
	let right_image = right_swapchain_images[self.right_image as usize];

. This is wasted work when we could be rendering.

[jdm]

This happened.