+/*! Create a new image cache. An image cache is basically a hash table to look
+ up RrImages. Each RrImage in the cache may contain one or more Pictures,
+ that is one or more actual copies of image data at various sizes. For eg,
+ for a window, all of its various icons are loaded into the same RrImage.
+ When an RrImage is drawn and a picture inside it needs to be resized, that
+ is also saved within the RrImage.
+
+ For each picture that an RrImage has, the picture is hashed and that is used
+ as a key to find the RrImage. So, given any picture in any RrImage in the
+ cache, if you hash it, you will find the RrImage.
+*/