Evas GL: Restore current context to evas gl context when the backend has taken over

Evas GL: Restore current context to evas gl context when the backend has taken over

Summary:
When Evas GL apis are called outside of on pixels callback,
evas gl backend context may have been made current, and Evas GL will
render into a wrong context.
So here we provide context restore mechanism of keeping track of
currently bound context and calling make current when needed.
@feature

Test Plan: Run Evas GL test cases

Reviewers: jpeg, cedric

Subscribers: mythri, mer.kim, wonsik, cedric

Differential Revision: https://phab.enlightenment.org/D2956