gpu: don't panic for nil op.Ops argument to GPU.Frame

Updates gio#306

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

gpu/compute.go

@@ -1723,7 +1723,11 @@ func (c *collector) addClip(state *encoderState, viewport, bounds f32.Rectangle,
 
 func (c *collector) collect(root *op.Ops, viewport image.Point, texOps *[]textureOp) {
 	fview := f32.Rectangle{Max: layout.FPt(viewport)}
-	c.reader.Reset(&root.Internal)
+	var intOps *ops.Ops
+	if root != nil {
+		intOps = &root.Internal
+	}
+	c.reader.Reset(intOps)
 	var state encoderState
 	reset := func() {
 		state = encoderState{

gpu/gpu.go

@@ -800,7 +800,11 @@ func (d *drawOps) collect(root *op.Ops, viewport image.Point) {
 	viewf := f32.Rectangle{
 		Max: f32.Point{X: float32(viewport.X), Y: float32(viewport.Y)},
 	}
-	d.reader.Reset(&root.Internal)
+	var ops *ops.Ops
+	if root != nil {
+		ops = &root.Internal
+	}
+	d.reader.Reset(ops)
 	d.collectOps(&d.reader, viewf)
 }
 

gpu/headless/headless.go

@@ -55,7 +55,7 @@ func newContext() (context, error) {
 	if firstErr != nil {
 		return nil, firstErr
 	}
-	return nil, errors.New("x11: no available GPU backends")
+	return nil, errors.New("headless: no available GPU backends")
 }
 
 // NewWindow creates a new headless window.

gpu/headless/headless_test.go

@@ -132,6 +132,14 @@ func TestDepth(t *testing.T) {
 	}
 }
 
+func TestNoOps(t *testing.T) {
+	w, release := newTestWindow(t)
+	defer release()
+	if err := w.Frame(nil); err != nil {
+		t.Error(err)
+	}
+}
+
 func newTestWindow(t *testing.T) (*Window, func()) {
 	t.Helper()
 	sz := image.Point{X: 800, Y: 600}