shiny/driver/windriver: use StretchBlt when AlphaBlend is not available

Fixes golang/go#15668

Change-Id: I482f3f068cad79351f45c91e77faa1ec5f3b6060
Reviewed-on: https://go-review.googlesource.com/23110
Reviewed-by: Nigel Tao <nigeltao@golang.org>
3 files changed