Blender Git Loki

Bugfix #27519

Full Sample AA (FSA) was failing in cases. Bug report was an empty
scene (with compo nodes) linking in another .blend scene (with render).

That case gave warning "FSA not supported with rendering". That now is
allowed.

Then I noticed FSA was giving corrupt sample buffers or crashes in cases,
especially on first buffer, this appeared to be a missing compo tag on
first sample buffer.

Lastly, to make FSA render a tiny bit less frustrating: added render window
statistic to show which of the FSA steps is being done.