Mark contexts as shared when importing EGL images. Once a context references an EGL image, it can share resources with contexts in other share groups. Marking the context as shared ensures that locks are held by all contexts referencing EGL images for GL functions. Bug: angleproject:6957 Change-Id: Ic3901c458f388306c59f6bb01560a7c66d0574c0 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3424659 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: mohan maiya <m.maiya@samsung.com> Commit-Queue: Geoff Lang <geofflang@chromium.org>