* libgimp/gimpzoompreview.[ch]: make use of private data instead of struct
  members. Added a gimp_zoom_preview_get_drawable so we can still get the
  drawable from a preview in plug-ins.

  Also changed gimp_zoom_preview_get_data to gimp_zoom_preview_get_source,
  to make it more obvious what it returns.

* libgimp/gimpui.def
* plug-ins/common/AlienMap2.c
* plug-ins/common/apply_lens.c
* plug-ins/common/blinds.c
* plug-ins/common/channel_mixer.c
* plug-ins/common/colorify.c
* plug-ins/common/flarefx.c
* plug-ins/common/illusion.c
* plug-ins/common/jigsaw.c
* plug-ins/common/mapcolor.c
* plug-ins/common/max_rgb.c
* plug-ins/common/nova.c
* plug-ins/common/polar.c
* plug-ins/common/retinex.c
* plug-ins/common/waves.c
* plug-ins/common/whirlpinch.c: changed accordingly.