Fix issue FS#255 : text background is redrawn when window size change.

Set `_affichage_depend_de_la_fenetre = True`.
Small fix for `convertir_en_LaTeX()`.

Empty string is converted to '', not '$$'.
Fix issue FS#234 : matplotlib doesn't support `\left]` nor `\right]`.

This is only a workaround, matplotlib itself should be patched instead.
Improve matplotlib 1.2 compatibility.