Blender Git Loki

Fixes for thread related render / compositing crashes:

* Viewer node could free image while it is being redrawn, viewer image
buffers now need acquire/release to be accessed as was already the
case for render results.
* The Composite node could free the image buffers outside of a lock,
also causing simultaneous redraw to crash.
* Especially on Windows, re-rendering could crash when drawing an image
that was freed. When RE_RenderInProgress was true it would access the
image buffer and simply return it while it could still contain a pointer
to a render result buffer that was already freed. I don't understand
why this case was there in the first place, so I've removed it.

Possibly fixes bugs #20174, #21418, #21391, #21394.