libsysprof-gtk: add a split layer for top/bottom rows

This can be used to recreate our old duplex visualizer which draws in
opposite directions from a common middle Y point.

src/libsysprof-gtk/sysprof-split-layer.h

@@ -0,0 +1,48 @@
+/*
+ * sysprof-split-layer.h
+ *
+ * Copyright 2023 Christian Hergert <chergert@redhat.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#pragma once
+
+#include "sysprof-chart-layer.h"
+
+#include <sysprof-analyze.h>
+
+G_BEGIN_DECLS
+
+#define SYSPROF_TYPE_SPLIT_LAYER (sysprof_split_layer_get_type())
+
+SYSPROF_AVAILABLE_IN_ALL
+G_DECLARE_FINAL_TYPE (SysprofSplitLayer, sysprof_split_layer, SYSPROF, SPLIT_LAYER, SysprofChartLayer)
+
+SYSPROF_AVAILABLE_IN_ALL
+SysprofChartLayer *sysprof_split_layer_new        (void);
+SYSPROF_AVAILABLE_IN_ALL
+SysprofChartLayer *sysprof_split_layer_get_bottom (SysprofSplitLayer *self);
+SYSPROF_AVAILABLE_IN_ALL
+void               sysprof_split_layer_set_bottom (SysprofSplitLayer *self,
+                                                   SysprofChartLayer *top);
+SYSPROF_AVAILABLE_IN_ALL
+SysprofChartLayer *sysprof_split_layer_get_top    (SysprofSplitLayer *self);
+SYSPROF_AVAILABLE_IN_ALL
+void               sysprof_split_layer_set_top    (SysprofSplitLayer *self,
+                                                   SysprofChartLayer *bottom);
+
+G_END_DECLS