add example which registers and uses go custom elements

this example has an assert that checks wether or nor the finalizers ran correctly. If they don't then a mem leak will happen

examples/plugins/registered_elements/main.go

@@ -0,0 +1,120 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/signal"
+	"path/filepath"
+	"runtime"
+	"runtime/pprof"
+
+	"github.com/go-gst/go-glib/glib"
+	"github.com/go-gst/go-gst/examples/plugins/registered_elements/internal/common"
+	"github.com/go-gst/go-gst/examples/plugins/registered_elements/internal/custombin"
+	"github.com/go-gst/go-gst/examples/plugins/registered_elements/internal/customsrc"
+	"github.com/go-gst/go-gst/gst"
+)
+
+func run(ctx context.Context) error {
+	ctx, cancel := signal.NotifyContext(ctx, os.Interrupt)
+	defer cancel()
+
+	wd, err := os.Getwd()
+
+	if err != nil {
+		return err
+	}
+
+	gst.Init(nil)
+
+	customsrc.Register()
+	custombin.Register()
+
+	systemclock := gst.ObtainSystemClock()
+
+	pipeline, err := gst.NewPipelineFromString("gocustombin ! fakesink sync=true")
+
+	if err != nil {
+		return err
+	}
+
+	pipeline.ForceClock(systemclock.Clock)
+
+	bus := pipeline.GetBus()
+
+	mainloop := glib.NewMainLoop(glib.MainContextDefault(), false)
+
+	pipeline.SetState(gst.StatePlaying)
+
+	bus.AddWatch(func(msg *gst.Message) bool {
+		switch msg.Type() {
+		case gst.MessageStateChanged:
+			old, new := msg.ParseStateChanged()
+			dot := pipeline.DebugBinToDotData(gst.DebugGraphShowVerbose)
+
+			f, err := os.OpenFile(filepath.Join(wd, fmt.Sprintf("pipeline-%s-to-%s.dot", old, new)), os.O_TRUNC|os.O_CREATE|os.O_RDWR, 0600)
+
+			if err != nil {
+				cancel()
+				return false
+			}
+
+			defer f.Close()
+
+			_, err = f.Write([]byte(dot))
+
+			if err != nil {
+				fmt.Println(err)
+				cancel()
+				return false
+			}
+
+		case gst.MessageEOS:
+			fmt.Println(msg.String())
+			cancel()
+			return false
+		}
+
+		// the String method is expensive and should not be used in prodution:
+		fmt.Println(msg.String())
+		return true
+	})
+
+	go mainloop.Run()
+
+	go func() {
+		<-ctx.Done()
+
+		mainloop.Quit()
+	}()
+
+	<-ctx.Done()
+
+	pipeline.BlockSetState(gst.StateNull)
+
+	gst.Deinit()
+
+	return ctx.Err()
+}
+
+func main() {
+	ctx := context.Background()
+
+	err := run(ctx)
+
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+	}
+
+	runtime.GC()
+	runtime.GC()
+	runtime.GC()
+
+	prof := pprof.Lookup("go-glib-reffed-objects")
+
+	prof.WriteTo(os.Stdout, 1)
+
+	// we are creating 3 custom elements in total. If this panics, then the go struct will memory leak
+	common.AssertFinalizersCalled(3)
+}