EGL: Validate that EGL_GL_TEXTURE_ZOFFSET is used only for 3D sources. BUG=angleproject:2507 Change-Id: I0e5879acfbb7ba1b9d711f518205cebad2edb643 Reviewed-on: https://chromium-review.googlesource.com/1148657 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org>