capture: add SpCaptureCursor and associated types

This API helps us simplify some of the tooling to iterate
through a capture. In particular, we might want to setup a
bunch of matches and then just iterate through the items.

This can also allow delaying the iteration until the future
which might be handy for visualizers which won't want to block
the main loop.

I'm not jazzed about the 64k buffer created for every cursor
due to the SpCaptureReader copy, but it's probably not a big
deal in practice until we start doing more exotic things.

tests/test-capture-cursor.c

@@ -0,0 +1,58 @@
+#include <glib/gstdio.h>
+#include <sysprof.h>
+
+static gboolean
+increment (const SpCaptureFrame *frame,
+           gpointer              user_data)
+{
+  gint *count= user_data;
+  (*count)++;
+  return TRUE;
+}
+
+static void
+test_cursor_basic (void)
+{
+  SpCaptureReader *reader;
+  SpCaptureWriter *writer;
+  SpCaptureCursor *cursor;
+  GError *error = NULL;
+  gint64 t = SP_CAPTURE_CURRENT_TIME;
+  guint i;
+  gint r;
+  gint count = 0;
+
+  writer = sp_capture_writer_new ("capture-file", 0);
+  g_assert (writer != NULL);
+
+  reader = sp_capture_reader_new ("capture-file", &error);
+  g_assert_no_error (error);
+  g_assert (reader != NULL);
+
+  for (i = 0; i < 100; i++)
+    {
+      r = sp_capture_writer_add_timestamp (writer, t, i, -1);
+      g_assert_cmpint (r, ==, TRUE);
+    }
+
+  sp_capture_writer_flush (writer);
+
+  cursor = sp_capture_cursor_new (reader);
+  sp_capture_cursor_foreach (cursor, increment, &count);
+  g_assert_cmpint (count, ==, 100);
+  g_clear_object (&cursor);
+
+  sp_capture_reader_unref (reader);
+  sp_capture_writer_unref (writer);
+
+  g_unlink ("capture-file");
+}
+
+int
+main (int argc,
+      char *argv[])
+{
+  g_test_init (&argc, &argv, NULL);
+  g_test_add_func ("/SpCaptureCursor/basic", test_cursor_basic);
+  return g_test_run ();
+}