gegl-scratch is a fast memory allocator, suitable for small (up to
a few megabytes), short-lived (usually, but not necessarily, bound
to the current stack-frame) allocations.  Unlike alloca(), gimp-
scratch doesn't use the stack, and is therefore safer, may outlive
the current stackframe, and will also serve bigger requests, by
falling-back to malloc().

The allocator itself is very simple:  We keep a per-thread stack of
cached memory blocks (allocated using the normal allocator).  When
serving an allocation request, we simply pop the top block off the
stack, and return it.  If the block is too small, we replace it
with a big-enough block; if the stack is empty, we allocate a new
block.  When the block is freed, we push it back to the top of the
stack.  The idea is that the stacks will ultimately stabalize to
contain blocks that can serve all the encountered allocation
patterns, without needing to reisze any of the blocks; as a
consequence, the amount of scratch memory a...