sw_engine: fix invalid data sharing at multi-threading.

sw_engine: fix invalid data sharing at multi-threading.

We have encountered that multi-threading usage that user creates,
multiple canvases owned by multiple user threads.

Current sw_engine memory pool has been considered only for multi-threads,
spawned by tvg task scheduler.

In this case it's safe but when user threads introduced, it can occur race-condition.

Thus, Here is a renewal policy that non-threading tvg(initialized threads with zero),
takes care of multiple user threads bu changing its policy,
each of canvases should have individual memory pool to guarantee mutual-exclusion.

@API additions

enum MempoolPolicy
{
    Default = 0, ///< Default behavior that ThorVG is designed to.
    Shareable,   ///< Memory Pool is shared among the SwCanvases.
    Individual   ///< Allocate designated memory pool that is only used by current instance.
};

Result SwCanvas::mempool(MempoolPolicy policy) noexcept;

All in all, if user calls multiple threads, set memory pool policy to Individual.