shiny/driver/windriver: fix destination rectangle formula

Fixes golang/go#19902

Change-Id: Iaa1218775830699865dc9c5613abf2e81362ed4d
Reviewed-on: https://go-review.googlesource.com/40891
Reviewed-by: Nigel Tao <nigeltao@golang.org>
diff --git a/shiny/driver/windriver/window.go b/shiny/driver/windriver/window.go
index cb12125..c66cd47 100644
--- a/shiny/driver/windriver/window.go
+++ b/shiny/driver/windriver/window.go
@@ -128,8 +128,7 @@
 		}()
 	} else if src2dst[0] == 1 && src2dst[4] == 1 {
 		// copy bitmap
-		dp := image.Point{int(src2dst[2]), int(src2dst[5])}
-		dr = sr.Add(dp.Sub(sr.Min))
+		dr = sr.Add(image.Point{int(src2dst[2]), int(src2dst[5])})
 	} else {
 		// scale bitmap
 		dstXMin := float64(sr.Min.X)*src2dst[0] + src2dst[2]