(bug 28430) Make HTML and TeX output of <math> tags be LTR always

(bug 28430) Make HTML and TeX output of <math> tags be LTR always

The PNG output of <math> is always LTR regardless of content language,
and since we switch back and forth between PNG/html depending on
how complicated the formula is, we should be consistent. Also according to
the bug always doing LTR even on RTL langs is appearently correct behaviour.

Also change for plain TeX output to be LTR, as otherwise the TeX commands
don't really make sense if $ ^{355} $ gets turned into $ {355}^ $