evas draw context - reduce size and allocations and frees

evas draw context - reduce size and allocations and frees

so a little perf fun shows malloc/free/realloc/etc. are, combined a
reasonable overhead. this reduced malloc overhead for draw contexts so
whne we duplicate them or create new ones, we re-use a small cache of
8 of them to avoid re-allocation. just take the first one from the
list as it really is that simple. mempool would not have helped more
here and cost more overhead.

@optimize