internal/graphicsdriver/metal: do not call PresentDrawable when CommandBuffer is empty

Before this fix, PresentDrawable could be invoked even when the
CommandBuffer was empty. This might cause the flickering issue #3278.

This is an experimental (but seemingly valid) fix.

Updates #3278

internal/graphicsdriver/metal/graphics_darwin.go

@@ -229,7 +229,10 @@ func (g *Graphics) SetVertices(vertices []float32, indices []uint32) error {
 }
 
 func (g *Graphics) flushIfNeeded(present bool) {
-	if g.cb == (mtl.CommandBuffer{}) && !present {
+	if g.cb == (mtl.CommandBuffer{}) {
+		if g.rce != (mtl.RenderCommandEncoder{}) {
+			panic("metal: command buffer is empty but render command encoder is not empty")
+		}
 		return
 	}