[jbig2dec] Custom allocator now supports test case being run more than once. (#5304)

To make that possible these things were also done:

 * Rename variables for clarity.
 * Rename functions to separate them from jbig2dec's own allocator API.
 * Consistently use size_t for all memory statistics.
 * Add helper function for checking peak memory use.
 * Add helper function for when limit is reached.
 * Add helper function for when allocations fail.
 * Add helper function for memory statis at end of run (including leak check).

This will help with fixing jbig2dec issue 21675 reported by OSS-fuzz.

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