This makes all debug options stored in a single struct rather than in
various places, changes/fixes the behaviour of existing options, and
introduces some new options.
* Fixes damage issues with `-Drender-tree` texture (by removing scissor)
* Offsets the render tree overlay's `y` position for those who have
swaybar at the top
* Replaces `-Ddamage=rerender` with `-Dnodamage`
* Replaces `-Ddamage=highlight` with `-Dhighlight-damage`
* Replaces `-Dtxn-debug` with `-Dtxn-wait`
* Introduces `-Dnoatomic`
* Removes the `create_time` and `ms_arranging` figures from transactions
and the log message. Transactions are created after arranging and the
create time is of no significance.
* Fixes `-Dtxn-debug` (now `-Dtxn-wait`) not working.
We currently have several ways of setting debug flags, including command
line arguments, environment variables, and compile-time macros. This
replaces the lot with command line flags.
