The "secret" the implementation is hiding is how calls and layers
turn into a single structure. Not a great secret but functionality
that is needed.
c/util: Use comp_layer_accum in comp_base and derived implmentations.
c/util: zero the other swapchains.
c/util: Inline two functions.
Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2317>