Function gvr_sys::gvr_buffer_viewport_get_source_fov

pub unsafe extern fn gvr_buffer_viewport_get_source_fov(viewport: *const gvr_buffer_viewport)
                                                        -> gvr_rectf

Retrieves the field of view for the referenced buffer region.

This is a helper that converts the stored projection matrix to a field of view. Note that if the previously set projection matrix cannot be expressed as a view frustum aligned with the eye's optical axis, the result will be incorrect.

@param viewport The buffer viewport. @return The field of view of the rendered image, in degrees.