This refactors output_ensure_buffer() to not mutate the state passed,
making the previous subtle behavior much more explicit.
Fixes: d483dd2f ("output: add wlr_output_commit_state")
Closes: #3442
This makes it possible for the two functions using output_pick_format
(output_pick_cursor_format and output_create_swapchain) to select
different buffer formats.
This organizes the wlr_output implementation into separate files.
This avoids having a single mega-file with lots of unrelated parts
and makes it more obvious what the interactions between all the
parts are.
No functional changes, just moving code around.