Should babel check if a direction switch is needed?

[Udi-Fogiel]

Currently Babel checks if \textdirection needs to be reassigned before doing so. This reduce the number of dir nodes created, which helped probably helped avoiding some bugs (see \breakafterdirmode and \mathemptydisplaymode for example).

This also has the following implication

\documentclass{article}
\usepackage[english,hebrew,bidi=default]{babel}

\begin{document}
\setbox0\hbox{\foreignlanguage{hebrew}{foo}}

\setbox1\hbox bdir0 {\foreignlanguage{hebrew}{foo}}

\selectlanguage{english}
\setbox2\hbox{\foreignlanguage{hebrew}{foo}}
with checking:

box0: \unhbox0\ box1: \unhbox1\ box2: \unhbox2

\vskip10pt
\selectlanguage{hebrew}
\makeatletter
\def\bbl@setluadir#1#2{#1=#2}
\setbox0\hbox{\foreignlanguage{hebrew}{foo}}

\setbox1\hbox bdir0 {\foreignlanguage{hebrew}{foo}}

\selectlanguage{english}
\setbox2\hbox{\foreignlanguage{hebrew}{foo}}
without checking:

box0: \unhbox0\ box1: \unhbox1\ box2: \unhbox2

\end{document}