pict: throw out LaTeX back-end and simplify

The "texpict" interface as based by LaTeX is ancient, undocumented,
and unlikely to have been used since the previous millennium. Throw it
out and simplify the implementation (by discarding signatures and
units).
pict: make picts serializable
scribble/eval: change handling of serializable convertible results

When an evaluation result from a sandboxed computation is convertible
(so that it should be preserved for rendering) and serializable, then
serialize the value from transfer from the sandboxed environment to
the rendering environment.

This change complements the one that makes `pict`s serializable.
Serializing a pict result avoids retaining the sandboxed environment,
including its instance of the `pict` and `racket/draw` modules,
when a sandbox-created pict is part of the generated document.
For example, the memory use of the result of "math.scrbl" is about
20% smaller with this change.
racket/port: fix busy-wait bug in `make-limited-input-port`

As reported by Marc Burns on the mailing list
fix interaction of `alarm-evt` and `replace-evt`

With `replace-evt` the time that the system needs to wake up
to check the event can drift later, but scheduling state was
carried in a way that works only if the wake-up time drifts
earlier.

Unfortunately, I don't know how to write a test for this bug.
The usual stategy of using `system-idle-evt` to detect busy
waiting doesn't work here, because the business happens despite
the scheduler's conclusion that the system is idle.

As reported by Jan Dvořák on the mailing list.
minor cleanup on thread termination