Fix problem that using multi-engines, sometimes OpenGL would crash be…

[wqyfavor]

…cause of invalid EAGLContext.

We are using multi-engines in our app. So there are chances that an iOS render target would be deallocated.

We found random crash and finally found that when ios_render_target_gl is being destructed.
The first line of this function "EAGLContext* context = EAGLContext.currentContext;". The context is actually equal to "context_.get()". So an invalid opengl context is set to current.

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[wqyfavor]

@googlebot I signed it!

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CLAs look good, thanks!

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[chinmaygarde]

ooh, thats subtle. Thanks for the fix. Can you clang-format this patch please? This is whats causing the presubmit failures. LGTM otherwise.

[chinmaygarde]

Thanks.