Add glib main loop to capture manager (#383)

The gstreamer documentation is not particularly amazing on whether or
not this is necessary, but it's clear that some gstreamer events will
not be delivered to their handlers without a running glib loop. This
runs one loop for all pipelines, which should be more than enough.

Disclaimer: This may conflict in demodesk/neko with the dragdrop
feature. Anyone backporting this bug fix to that repo should
investigate whether the loop created by `gtk_main()` will conflict with
this one before blindly porting.

Fixes #380
Fixes #284

server/internal/capture/gst/gst.go

@@ -31,12 +31,29 @@ var pSerial int32
 var pipelines = make(map[int]*Pipeline)
 var pipelinesLock sync.Mutex
 var registry *C.GstRegistry
+var gMainLoop *C.GMainLoop
 
 func init() {
 	C.gst_init(nil, nil)
 	registry = C.gst_registry_get()
 }
 
+func RunMainLoop() {
+	if gMainLoop != nil {
+		return
+	}
+	gMainLoop = C.g_main_loop_new(nil, C.int(0))
+	C.g_main_loop_run(gMainLoop)
+}
+
+func QuitMainLoop() {
+	if gMainLoop == nil {
+		return
+	}
+	C.g_main_loop_quit(gMainLoop)
+	gMainLoop = nil
+}
+
 func CreatePipeline(pipelineStr string) (*Pipeline, error) {
 	id := atomic.AddInt32(&pSerial, 1)