New rendering option: FSA!

This completes the pipeline make-over, as started in 2006. With this
option, during rendering, each sample for every layer and pass is being
saved on disk (looks like non-antialiased images). Then the composite 
and color correction happens, then a clip to 0-1 range, and only in end 
all samples get combined - using sampling filters such as gauss/mitch/catmul.

This results in artefact-free antialiased images. Even Z-combine or
ID masks now work perfect for it! 

This is an unfinished commit btw; Brecht will finish this for strands.
Also Halo doesnt work yet.

To activate FSA: press "Save Buffers" and the new button next to it. :)

source/blender/render/intern/include/rendercore.h

@@ -93,6 +93,9 @@ void zbufshade_sss_tile(struct RenderPart *pa);
 
 void addps(struct ListBase *lb, long *rd, int obi, int facenr, int z, unsigned short mask);
 
+int get_sample_layers(struct RenderPart *pa, struct RenderLayer *rl, struct RenderLayer **rlpp);
+
+
 /* -------- ray.c ------- */
 
 extern void freeraytree(Render *re);