evas gl engines - avoid getting context if possible

evas gl engines - avoid getting context if possible

so getting context at least on some dviers is expensive. it may really
impact cpu usage a lot (in this cate getpid() was being called by the
nouveau drivers and that can be expensive. it is on ARM as it's a full
syscall and 1-2% of cpu time was just getting pid all the time thanks
to this...

@opt