[XeTeX] additional beginL endL nodes in math

[David Carlisle]

As noted in the release notes direction support now works in math
which is a good thing but a side effect seems to be that beginL endL
nodes are added to every math list

\tracingoutput1
\tracingonline1

$a$

\bye


generates

..\hbox(4.30554+0.0)x469.75499, glue set 444.4691fil
...\hbox(0.0+0.0)x20.0
...\mathon
...\beginL
...\teni a
...\endL
...etc.
..\glue 0.0 plus 1.0fill
.\glue(\baselineskip) 17.55556


This seems quite an obtrusive change for documents not using explicit
directionality (and causes 77 tests in the latex regression test suite
to fail in 2015 pretest)

Would it be possible for the automatic beginL node _not_ to be added
if the current context was already left to right?

David