[client] app: implement new overlay rendering framework

This change set implements a framework for overlays to be registered
that make use of ImGui. See `overlay/fps` for a simple implementation
example.

client/src/overlay/fps.c

@@ -0,0 +1,79 @@
+/**
+ * Looking Glass
+ * Copyright (C) 2017-2021 The Looking Glass Authors
+ * https://looking-glass.io
+ *
+ * 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 2 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, write to the Free Software Foundation, Inc., 59
+ * Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#include "interface/overlay.h"
+#include "cimgui.h"
+
+#include "../main.h"
+
+static bool fps_init(void ** udata, void * params)
+{
+  return true;
+}
+
+static void fps_free(void * udata)
+{
+}
+
+static int fps_getWindowCount(void * udata, bool interactive)
+{
+  return g_state.showFPS ? 1 : 0;
+}
+
+static void fps_render(void * udata, bool interactive, struct Rect * windowRects)
+{
+  const ImVec2 pos = {0.0f, 0.0f};
+  igSetNextWindowBgAlpha(0.6f);
+  igSetNextWindowPos(pos, 0, pos);
+
+  igBegin(
+    "FPS",
+    NULL,
+    ImGuiWindowFlags_NoDecoration    | ImGuiWindowFlags_AlwaysAutoResize   |
+    ImGuiWindowFlags_NoSavedSettings | ImGuiWindowFlags_NoFocusOnAppearing |
+    ImGuiWindowFlags_NoNav           | ImGuiWindowFlags_NoTitleBar
+  );
+
+  igText("FPS:%4.2f UPS:%4.2f",
+      atomic_load_explicit(&g_state.fps, memory_order_relaxed),
+      atomic_load_explicit(&g_state.ups, memory_order_relaxed));
+
+  if (windowRects)
+  {
+    ImVec2 pos, size;
+    igGetWindowPos(&pos);
+    igGetWindowSize(&size);
+    windowRects[0].x = pos.x;
+    windowRects[0].y = pos.y;
+    windowRects[0].w = size.x;
+    windowRects[0].h = size.y;
+  }
+
+  igEnd();
+}
+
+struct LG_OverlayOps LGOverlayFPS =
+{
+  .name           = "FPS",
+  .init           = fps_init,
+  .free           = fps_free,
+  .getWindowCount = fps_getWindowCount,
+  .render         = fps_render
+};