cmd/gogio/testdata: always clear background to white

The webassembly port changed to use transparent black to allow content
below Gio to show through. However, the end-to-end tests expect a white
background.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

cmd/go.mod

@@ -3,7 +3,7 @@ module gioui.org/cmd
 go 1.13
 
 require (
-	gioui.org v0.0.0-20201206220230-a87a520ae825
+	gioui.org v0.0.0-20210218074926-aaff0fef5381
 	github.com/akavel/rsrc v0.10.1
 	github.com/chromedp/cdproto v0.0.0-20191114225735-6626966fbae4
 	github.com/chromedp/chromedp v0.5.2

cmd/go.sum

@@ -1,6 +1,6 @@
 dmitri.shuralyov.com/gpu/mtl v0.0.0-20190408044501-666a987793e9/go.mod h1:H6x//7gZCb22OMCxBHrMx7a5I7Hp++hsVxbQ4BYO7hU=
-gioui.org v0.0.0-20201206220230-a87a520ae825 h1:8eQeFlQ0IL5sOX74YcwEBk3OtGNTRCqIU3Rz0z0U6vE=
-gioui.org v0.0.0-20201206220230-a87a520ae825/go.mod h1:Y+uS7hHMvku1Q+ooaoq6fYD5B2LGoT8JtFgvmYmRzTw=
+gioui.org v0.0.0-20210218074926-aaff0fef5381 h1:g+ptkCry6O5+guP707JpLuCY48TtAtrTWOB5qlzw4VM=
+gioui.org v0.0.0-20210218074926-aaff0fef5381/go.mod h1:Y+uS7hHMvku1Q+ooaoq6fYD5B2LGoT8JtFgvmYmRzTw=
 github.com/BurntSushi/xgb v0.0.0-20160522181843-27f122750802/go.mod h1:IVnqGOEym/WlBOVXweHU+Q+/VP0lqqI8lqeDx9IjBqo=
 github.com/akavel/rsrc v0.10.1 h1:hCCPImjmFKVNGpeLZyTDRHEFC283DzyTXTo0cO0Rq9o=
 github.com/akavel/rsrc v0.10.1/go.mod h1:uLoCtb9J+EyAqh+26kdrTgmzRBFPGOolLWKpdxkKq+c=

cmd/gogio/testdata/testdata.go

@@ -67,6 +67,8 @@ func loop(w *app.Window) error {
 			return e.Err
 		case system.FrameEvent:
 			gtx := layout.NewContext(&ops, e)
+			// Clear background to white, even on embedded platforms such as webassembly.
+			paint.Fill(gtx.Ops, color.NRGBA{A: 0xff, R: 0xff, G: 0xff, B: 0xff})
 			layout.Flex{Axis: layout.Vertical}.Layout(gtx,
 				layout.Flexed(1, func(gtx C) D {
 					return layout.Flex{Axis: layout.Horizontal}.Layout(gtx,