locate_views calls xrLocateViews twice

[jdm]

The openxr crate uses the get_arr_init helper in locate_views. This means that it calls it first to determine how many views there are, then it calls it again with a vector of the right size to receive the view information. The XR sample app preallocates the right number of views an only calls it once every frame. We should look into whether we can do the same by dipping down into the system openxr crate.

[Manishearth]

We can also update that code to determine the length based on the ViewConfigurationType

(We can also also have it return an enum of arrays instead so it doesn't allocate)

[Manishearth]

Maybe better would be a locate_views_stereo() function (and a mono() one if people ask for it)