capture: add simple mark support

The goal here is to have an API that allows us to record things like
frame timing data. We might iterate on this API a bit, but this gets us
started.

A SpCaptureMark with a zero duration should be treated like an epoch mark
once a visualizer is created. SpCaptureMark with a non-zero duration should
be treated like a begin/end of operation. This may be useful in generating
something like a flame graph.

lib/capture/sp-capture-cursor.c

@@ -99,6 +99,10 @@ sp_capture_cursor_foreach (SpCaptureCursor         *self,
           delegate = READ_DELEGATE (sp_capture_reader_read_map);
           break;
 
+        case SP_CAPTURE_FRAME_MARK:
+          delegate = READ_DELEGATE (sp_capture_reader_read_mark);
+          break;
+
         case SP_CAPTURE_FRAME_PROCESS:
           delegate = READ_DELEGATE (sp_capture_reader_read_process);
           break;