Blender Git Loki

Compositing workflow upgrade;

You now can set a Preview panel in the Image window, to define a sub-rect
of an image to be processed. Works like the preview in 3D Window. Just
press SHIFT+P to get it activated. Very nice speedup!

This is how it works:

- The compositor still uses the scene image size (including % setting) for
Viewer or Composite output size
- If a preview exists, it calculates the cropped rect from its position
in the Image window, and stores that in the Scene render data
- On composite execute, it copies only this part from the 'generator nodes',
right now Images or Render Results. That makes the entire composite tree
only using small rects, so it will execute fast.
- Also the render window will only display the cropped rect, and on F12
only the cropped part is being executed
- On rendering in background mode, the cropping is ignored though.

Usability notes:

- translating or zooming view will automatically invoke a recalculation
- if you zoom in on details, the calculated rect will even become smaller
- only one Imagewindow can have this Preview Panel, to prevent conflicts of
what the cropped area should be. Compositing is on Scene level, not local
per image window. (Note; 3D Previews are local per window!)
- Closing the preview panel will invoke a full-size recalculation
- All passes/layers from rendering are nicely cropped, including Z and
vectors.

The work to make the compositor do cropping was simple, but getting the
Image window displaying correctly and get all events OK was a lot of work...
indeed, we need to refactor Image Window usage once. Sorry for making the
mess even bigger now. :) I've tried not to interfere with UV edit or Paint
though... only when you're in compositing mode the panel will work.

BUG fix:

3D Preview render didn't work when multiple layers were set in the current
scene.