Cycles preview rendering could free the image buffers being used by drawing in
another thread due to a race condition. This race condition was unlikely before,
but now that preview renders are started right before we draw the image in the
image editor or load it as a texture in the 3D viewport, it's likely to happen.
As we are close to release this is too risky to fix properly, just avoid freeing
the cache for preview renders instead and accept increased memory usage in some
cases.
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