shiny/driver/windriver: share some code in window Upload, Fill and Draw

No functional changes. Just some code code shuffling.

Change-Id: Iac6e431f279ec37e7ae0f5131ee5581280f098a2
Reviewed-on: https://go-review.googlesource.com/21855
Reviewed-by: David Crawshaw <crawshaw@golang.org>
1 file changed