Simply add an option to render settings to save an EXR cache,
just when the render is finished. Also changed RE_ReadRenderResult() to read
cache instead of temp sample files (those are fully volatile now anyway).

Path to save cached render results is an UserPreferences setting.

Also added 'Reload render' feature to the Image Editor (so one can now re-open a blend,
and in an Image Editor hit ctrl-R to (try to) reload last render from cache).

Reviewers: campbellbarton, sergey

Differential Revision: https://developer.blender.org/D553