Release the current context's surface before making the next context current. We were calling context->releaseSurface on a context that was no longer current, this somtimes caused incorrect framebuffers to be deleted. BUG=angleproject:2464 Change-Id: I6859b5739a509c1343901f8c0eef1fdd1598cea1 Reviewed-on: https://chromium-review.googlesource.com/1108087 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>