Use a document plus symbol id for the image caching key
The use of a static global collection of image caches requires the use of a more unique identifier than just the symbol id which will often have caching id clashes.
Fixes: #4156 (closed)
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The use of a static global collection of image caches requires the use of a more unique identifier than just the symbol id which will often have caching id clashes.
Fixes: #4156 (closed)