Genie:  Allocate in BitMap_bounds::Set(), always

Previously on bounds-set we'd only reallocate an existing bitmap buffer.
Now, we always allocate.  This saves noninteractive users from having to
wait for a window update to allocate the buffer in Draw().
Nitrogen:  Move Ptr to Mac/Memory/Types/Ptr.hh
[NEW] MacVFS:  Add Ptr_memory_mapping
Genie:  Make bitmap/bits mmap-able
Merge branch 'bitmap-bits-mmap'