Highlight currently rendering tiles

This commit implements highlight of tiles which are being currently
rendered for both Blender Internal and Cycles (and should be possible
to use it for other external engines as well).

Couple of implementation details:

- Added one extra boolean flag to render engine which should be set
  to truth if render engine wants to highlight tiles. If so, property
  use_highlight_tiles should be set to True.

- Render Part's ready boolena was changed by status enum, which could
  be NONE, IN_PROGRESS and READY. All render part with IN_PROGRESS
  status will be highlighted in image editor.

- For external engines render part's status is filling in automatically.
  Initially all render parts has got NONE status, then one external
  engine acquire render result, corresponding part will change status
  to IN_PROGRESS. As soon as render result is finished, corresponding
  render part will change status to FINISHED

  This should make it easy to highlight tiles for other engines as well.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53486 954f8c5b-7b00-dc11-b283-0030488c597c