WIP: renderer-native: Simplify page flip counting.
Eliminate two fields of
This is necessary to support future asynchronous page flipping in which
a condition we currently test for
total_pending_flips == 0 might never
be true during constant rendering.
Turns out we already kept enough counting information in
pending_..._notify_frame_count to not need the deleted fields anyway.
So instead of waiting for
total_pending_flips == 0, we only need to wait
for specific invocations of
flip_closure_destroyed where the two counts
are different to know that a global frame has been completed for all CRTCs.
This was only intended to unblock work on !73 (closed), but has two additional unexpected happy consequences:
- fb_in_use is now truly unused and can be removed; and
- this is less code.