First step in order to solve #425.
I've replaced some constant images and their bounds with two map caches
and the associated map of bounds. The map of bounds is not really
satisfying as it relies on border effects, but it was fatster to
implement...
I still must be sure the cache is emptied when needed and that potential
equivalent features are well managed too.
Includes VariableOnlineResource in the Symbolizing tree.
Cache is purged when the underlying values change.
In order to acieve that, I've ahd to check update and setParent are
correctly set in Parameters.
Merge pull request #430 from agueganno/fix-425-ExternalGraphics

Improve management of external graphics