Merges the 3 threading parameters into a single `--threads=` argument.
Frame threading can still be controlled via the `--framedelay=` argument.
Internally, the threading model is now a global thread/task pool design.
Co-authored-by: Ronald S. Bultje <rsbultje@gmail.com>
A symbol starting with two leading underscores is reserved for
the compiler/standard library implementation.
Also remove the trailing two double underscores for consistency
and symmetry.