Using a RenderResult (or a Viewer) was never really working (think you
cant get a real ImBuff from these) -- cannot use it as a clone, stencil
or canvas [Single Image paint texture slot].
In the case of using it as a 2D paint clone image this would also crash
[due to the Image Editor drawing refactor in 2.91].
Now [in the spirit of T73182 / D11179], prevent using these where
unappropriate by using rna pointer polling functions.
Also add a security check for the 2D paint clone image crash in case a
stencil ImBuff cannot be provided for some reason, but generally old
files are now patched in do_versions_after_linking_300 (thx @brecht!).
Fixes T91625.
Maniphest Tasks: T91625
Differential Revision: https://developer.blender.org/D12609
fc2255135e