imageCacheObj keys need to be expanded

[sdlime]

The keys used to compare images stored in a symbolSetObj's image cache are not 
complete enough. At present only symbol, size and color are used. This will 
work in most cases, especially with lines which is where the cache is used now. 
However, symbol styles are more complex so most other attributes (e.g. 
outlinecolor) should be used. This will have to be the case if the cache is to 
be used with point markers. That's probably why the code is at it is...

Steve