Validate buffers bound for transform feedback are not modified. The ES spec says it is undefined to write to a buffer that is currently being used for transform feedback output but recommends generating an error. Generate INVALID_OPERATION in this case. Bug: chromium:427162086 Change-Id: I727d18c2035509fe2e5d60680eb5198e40a60e33 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6673310 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>