[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/graphs.c

@@ -0,0 +1,175 @@
+/**
+ * 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"
+
+#include "ll.h"
+#include "common/debug.h"
+
+struct GraphState
+{
+  struct ll * graphs;
+};
+
+static struct GraphState gs = {0};
+
+struct OverlayGraph
+{
+  const char * name;
+  RingBuffer   buffer;
+  bool         enabled;
+};
+
+static bool graphs_init(void ** udata, void * params)
+{
+  gs.graphs = ll_new();
+  return true;
+}
+
+static void graphs_free(void * udata)
+{
+  ll_free(gs.graphs);
+}
+
+static int graphs_getWindowCount(void * udata, bool interactive)
+{
+  return g_state.showTiming ? 1 : 0;
+}
+
+struct BufferMetrics
+{
+  float min;
+  float max;
+  float sum;
+  float avg;
+  float freq;
+};
+
+static bool rbCalcMetrics(int index, void * value_, void * udata_)
+{
+  float * value = value_;
+  struct BufferMetrics * udata = udata_;
+
+  if (index == 0)
+  {
+    udata->min = *value;
+    udata->max = *value;
+    udata->sum = *value;
+    return true;
+  }
+
+  if (udata->min > *value)
+    udata->min = *value;
+
+  if (udata->max < *value)
+    udata->max = *value;
+
+  udata->sum += *value;
+  return true;
+}
+
+static void graphs_render(void * udata, bool interactive, struct Rect * windowRects)
+{
+  const ImVec2 pos = {0.0f, 0.0f};
+  igSetNextWindowBgAlpha(0.4f);
+  igSetNextWindowPos(pos, 0, pos);
+
+  igBegin(
+    "Performance Metrics",
+    NULL,
+    ImGuiWindowFlags_NoDecoration    | ImGuiWindowFlags_AlwaysAutoResize   |
+    ImGuiWindowFlags_NoSavedSettings | ImGuiWindowFlags_NoFocusOnAppearing |
+    ImGuiWindowFlags_NoNav           | ImGuiWindowFlags_NoTitleBar
+  );
+
+  GraphHandle graph;
+  for (ll_reset(gs.graphs); ll_walk(gs.graphs, (void **)&graph); )
+  {
+    if (!graph->enabled)
+      continue;
+
+    struct BufferMetrics metrics = {};
+    ringbuffer_forEach(graph->buffer, rbCalcMetrics, &metrics);
+
+    if (metrics.sum > 0.0f)
+    {
+      metrics.avg  = metrics.sum / ringbuffer_getCount(graph->buffer);
+      metrics.freq = 1000.0f / metrics.avg;
+    }
+
+    char  title[64];
+    const ImVec2 size = {400.0f, 100.0f};
+
+    snprintf(title, sizeof(title),
+        "%s: min:%4.2f max:%4.2f avg:%4.2f/%4.2fHz",
+        graph->name, metrics.min, metrics.max, metrics.avg, metrics.freq);
+
+    igPlotLinesFloatPtr(
+        "",
+        (float *)ringbuffer_getValues(graph->buffer),
+        ringbuffer_getLength(graph->buffer),
+        ringbuffer_getStart (graph->buffer),
+        title,
+        0.0f,
+        50.0f,
+        size,
+        sizeof(float));
+  };
+
+  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 LGOverlayGraphs =
+{
+  .name           = "Graphs",
+  .init           = graphs_init,
+  .free           = graphs_free,
+  .getWindowCount = graphs_getWindowCount,
+  .render         = graphs_render
+};
+
+GraphHandle overlayGraph_register(const char * name, RingBuffer buffer)
+{
+  struct OverlayGraph * graph = malloc(sizeof(struct OverlayGraph));
+  graph->name    = name;
+  graph->buffer  = buffer;
+  graph->enabled = true;
+  ll_push(gs.graphs, graph);
+  return graph;
+}
+
+void overlayGraph_unregister(GraphHandle handle)
+{
+  handle->enabled = false;
+}