Previously there was only one buffer which contained all the
tiles currently visible. This is inflexible - we need to control
each tile individually. This commit introduces SubTile object,
which is the holder of the buffer for each individual tile.
Change-Id: I511f13dc7fad7c3c04f3d7f23b3abc97a3cc2268
Tomaž Vajngerl
3799a6f3a8