Storage hierarchy
1. URL_open
   Format: raw data
   Size: limited by XXX
   Speed: slow, often non-deterministic, 
   Notes: This accesses remote stuff, most often places it into the resource cache

2. Resource cache
   Format: raw data
   Size: controlled by XXX
   Speed: deterministic if item is wholly in cache, faster than #1
   Notes: storage place for resources 
   Source: #1

3. Decompressed image cache
   Format: RGB pixel format
   Size : controlled by XXX
   Notes: Image info is decompressed into a single format; image size usually
      increases (=width*height*3).  Note that a major improvement would be to
      flush the item from the resource cache when this is done decompressing;
      or to tune this cache for a fairly small size.
   Source: (#1 or #2) and processed by image decompressor
   Speed: much faster than #2


4. Page render cache
   Format: text lines
   Size: 1 page?
   Notes: used to redraw window during scrolling, etc.
   Source: (#1 or #2) and processed by format interpreter
   Speed: roughly the same as #1 for plain text, faster than that if HTML due
    to interpretor overhead


