app: add gimp-scratch allocator
gimp-scratch is a fast memory allocator (on the order of magnitude of alloca()), suitable for small (up to a few megabytes), short- lived (usually, bound to the current stack-frame) allocations. Unlike alloca(), gimp-scratch doesn't use the stack, and is therefore safer, 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. When the block is freed, we push it back to the top of the stack (note that even though each thread uses a separate stack, blocks can be migrated between threads, i.e., allocated on one thread, and freed on another thread, although this is not really an intended usage pattern.) 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 allocated at any given time should really be kept to a minimum.
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