This is the result of an audit of all static variables, looking

This is the result of an audit of all static variables, looking
for possible thread conflicts, of the sort Georg resolved at
6a30211f. I have made static variables const where possible,
and marked cases that looked potentially problematic with the
comment:
	// FIXME THREAD
Many of these definitely are vulnerable to concurrent access, such
as the static variables declared at the start of output_latex.cpp.
Suppose, e.g., we were outputting latex and also displaying the
source of a different document.

I'd appreciate it if others could grep for "FIXME THREAD" and see
if some of these are harmless, or what.