* libgimpwidgets/gimppreviewarea.c
* libgimpwidgets/gimppreviewarea.h: added two functions:
  gimp_preview_area_blend() to draw the blending of two buffers with
  an opacity parameter, and gimp_preview_area_mask() to draw the
  blending of two buffers, with a mask buffer. The code still needs some
  polish, though.

* libgimp/gimpdrawablepreview.c
* libgimp/gimpdrawablepreview.h: use gimp_preview_area_mask() in
  gimp_drawable_preview_draw(), so the previews are now much more
  accurate (respecting the selection, if any).

Also made the buf parameter of gimp_drawable_preview_draw() a pointer
to constants.