XR_KHR_composition_layer_equirect2 ambiguities

The mapping of the swapchain subimage to the inside surface of the partial sphere by XR_KHR_composition_layer_equirect2 is ambiguous. It could benefit from a diagram as provided for XR_KHR_composition_layer_cylinder.

For example, some questions:
* what direction is the "0 radian angle" of the partial sphere when angles are less than their maximum extents?
* Towards negative Z of the pose?
* what should the extremities of provided angles (corners of partial sphere) map to?
* The corners of the swapchain subimage (so the subimage fills entire partial sphere)?
* The mapped coordinates assuming a full sphere, i.e. based on the vector from the pose (such that the partial sphere shows only a window into the equirectangular subimage)?
* what direction should the center U coordinate of the swapchain subimage be mapped?
* Towards the "0 radian angle"?
* Fixed relative to the space?

Monado currently doesn't scale, position or orient the partial sphere or the texture mapping at all (and the points on the partial sphere map directly to the subimage based on the direction rather than the angle within the partial sphere), so I'm left guessing what the correct behaviour was intended to be.

Start with the XR_KHR_composition_layer_equirect2 specification and compare its mapping description with the diagram for XR_KHR_composition_layer_cylinder. Review the listed questions and the existing comment discussion to establish the intended angle, subimage, and partial-sphere mapping. Done means the specification is unambiguous and includes an explanatory diagram or equivalent guidance.

Written by the indexing model from the issue text.