Fix issues with calling View2D zoom operators in an `EXEC` context

Multiple related issues:
* `view_zoomdrag_exec()` required `view_zoomdrag_init()` to be called first to
  set up data. Can now easily be done in the `_exec()` function as well though.
* Because of that the operators could not be called in an `EXEC_` context (e.g.
  `EXEC_REGION_WIN`).
* With "Zoom to Mouse Position" enabled, zooming from a menu or script would
  zoom to the mouse position. That should only happen if the operators are
  called directly in the editor.