* do not lookup the same macro all the time

* do not lookup the same macro all the time
* only update the representation if anything was changed (this gives a
  huge speedup), fixes #2452
* minor cleanup of the code, especially setting up the coordinate
  cache. Without this can lead to crashes if the macros do not mention
  all the arguments.
* And a last fix included makes sure that the metrics are always in
  sync with the drawing. Before it was possible to go into the macro
  with the cursor in a way that the metrics were for the viewing mode,
  but the drawing was done for editing.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@18489 a592a061-630c-0410-9148-cb99ea01b6c8