gpu: don't automatically clear screen before rendering

Gio UI may be overlaid on top of custom graphics such as in the glfw example.
That will only work if Gio doesn't clear the screen (to white).

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

app/headless/headless.go

@@ -6,6 +6,7 @@ package headless
 
 import (
 	"image"
+	"image/color"
 	"runtime"
 
 	"gioui.org/gpu"
@@ -108,6 +109,7 @@ func (w *Window) Release() {
 // operation list.
 func (w *Window) Frame(frame *op.Ops) error {
 	return contextDo(w.ctx, func() error {
+		w.gpu.Clear(color.NRGBA{A: 0xff, R: 0xff, G: 0xff, B: 0xff})
 		w.gpu.Collect(w.size, frame)
 		return w.gpu.Frame()
 	})