screen: update EndPaint with generation number

Matches x/mobile change in http://golang.org/cl/12841.

Change-Id: If06e8addb126666820b02d2f2f34737204fbf281
Reviewed-on: https://go-review.googlesource.com/12925
Reviewed-by: Nigel Tao <nigeltao@golang.org>
4 files changed