Orange; more render & compo stuff!

-> Rendering in RenderLayers

It's important to distinguish a 'render layer' from a 'pass'. The first is
control over the main pipeline itself, to indicate what geometry is being
is rendered. The 'pass' (not in this commit!) is related to internal
shading code, like shadow/spec/AO/normals/etc.

Options for RenderLayers now are:
- Indicate which 3d 'view layers' have to be included (so you can render
  front and back separately)
- "Solid", all solid faces, includes sky at the moment too
- "ZTransp", all transparent faces
- "Halo", the halos
- "Strand", the particle strands (not coded yet...)

Currently only 2 'passes' are exported for render, which is the "Combined"
buffer and the "Z. The latter now works, and can be turned on/off.

Note that all layers are still fully kept in memory now, saving the tiles
and layers to disk (in exr) is also todo.

-> New Blur options

The existing Blur Node (compositor) now has an optional input image. This
has to be a 'value buffer', which can be a Zbuffer, or any mask you can
think of. The input values have to be in the 0-1 range, so another new
node was added too "Map Value".
The value input can also be used to tweak blur size with the (todo)
Time Node.

Temporal screenies:
http://www.blender.org/bf/rt.jpg
http://www.blender.org/bf/rt1.jpg
http://www.blender.org/bf/rt2.jpg

BTW: The compositor is very slow still, it recalulates all nodes on each
change still. Persistant memory and dependency checks is coming!

source/blender/include/butspace.h

@@ -319,8 +319,9 @@ void curvemap_buttons(struct uiBlock *block, struct CurveMapping *cumap, char la
 #define B_SWITCHRENDER		1641
 #define B_FBUF_REDO		1642
 
-#define B_SET_EDGE		1643
-#define B_SET_ZBLUR		1644
+#define B_SET_EDGE			1643
+#define B_SET_ZBLUR			1644
+#define B_ADD_RENDERLAYER	1645
 
 /* *********************** */
 #define B_ARMATUREBUTS		1800