It is still possible to free a bit more memory by detecting builtin images
which are not used by shaders, but that's not going to improve memory usage
that much to bother about this now.
Such change brings peak memory usage from 4.1GB to 3.4GB when rendering
01_01_01_D layout scene from the Gooseberry project. Mainly because of
freeing memory used by rather huge environment map in the viewport.