Make the export of macros thread-safer

Make the export of macros thread-safer

When we export the file to latex, we use the redefinition_ variable to check whether we should output newcommand or renewcommand. This variable was set by the MathMacroTemplate::metrics() function, and this caused problem when the export is running in a different thread as the GUI.

In general, the metrics() functions should not change the Buffer; we have updateBuffer/updateMacros for that purpose.