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/loop.go

@@ -4,6 +4,7 @@ package app
 
 import (
 	"image"
+	"image/color"
 	"runtime"
 
 	"gioui.org/app/internal/window"
@@ -85,6 +86,7 @@ func (l *renderLoop) renderLoop(ctx window.Context) error {
 				l.refreshErr <- ctx.MakeCurrent()
 			case frame := <-l.frames:
 				ctx.Lock()
+				g.Clear(color.NRGBA{A: 0xff, R: 0xff, G: 0xff, B: 0xff})
 				g.Collect(frame.viewport, frame.ops)
 				// Signal that we're done with the frame ops.
 				l.ack <- struct{}{}