ui/app/internal/gpu: set the default material to black

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

ui/app/internal/gpu/gpu.go

@@ -688,6 +688,7 @@ func (d *drawOps) collect(cache *resourceCache, root *ui.Ops, viewport image.Poi
 	state := drawState{
 		clip:  clip,
 		rect:  true,
+		color: color.RGBA{A: 0xff},
 	}
 	d.collectOps(&d.reader, state)
 }