Use the GL_COPY_READ_BUFFER binding for copy sources. Using the ELEMENT_ARRAY binding can cause issues such as rebinding the current VAO's index buffer unexpectedly. BUG=angleproject:881 Change-Id: Ieeed52ac81c23a46bfe9fc4365239c2b177ff6ac Reviewed-on: https://chromium-review.googlesource.com/261563 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>