Rebuilds any caches associated with the bitmap that are used for drawing it. In the case of purgeable bitmaps, this call will attempt to ensure that the pixels have been decoded. If this is called on more than one bitmap in sequence, the priority is given in LRU order (i.e. the last bitmap called will be given highest priority). For bitmaps with no associated caches, this call is effectively a no-op, and therefore is harmless.