Add gegl-tile-alloc -- a dedicated allocator for tile data.  The
allocator amortizes the cost of memory allocation, which is
especially high on Windows, over multiple tiles, by allocating
tiles in bulk, in groups called blocks (currently, each block is
roughly 1% of the tile-cache size.)

The allocator is currently limited to tiles whose size is a power
of 2, possibly multiplied by 3 or 5 (to support tiles whose
dimensions are powers of 2, with up to 5 components).  The rest of
the tiles are allocated using the normal allocator.

Move the call to malloc_trim() from GeglBuffer's destructor to
gegl-tile-alloc.  We call it whenever tile data equivalent to ~10%
of the tile-cache size is freed.

Add a tile-alloc-total stat, which reports the total amount of
memory currently used by the tile allocator.