skybox/equirect: correct radius to cover all visible area

monado now obeys the "radius" parameter for equirect layers, meaning the current 1.f value creates a small sphere rather than the original (intended) behavior of acting as a skybox.

Per the OpenXR specs, both zero and infinite values for radius will result in an infinitely-sized sphere, so we adopt that here.

It seems like this was already done in !16 (closed), but the branch no longer exists so isn't able to be referenced.

Old Behavior