Cache::Clear : workaround for gcc 4.7.2

without "this->" compiler see "size" readonly, but not "unallocated_list" line 218, surprising ...
this is the error :
Util/Cache.hpp:212:16: error: decrement of read-only location ‘this->Cache<Key, Data, capacity, Hash, Equal>::size’
Fix PNA & PC Broken Build.
Fix buffer underflow read.
import xcsoar RawBitmap for draw terrain.