[Add] Debug overlay and UI fixes.

- Real time on-screen debug overlay to display performance metrics.
  Overlay can be toggled with C-d.

[Fixed]
- Corrects a "click-through" bug by implementing Z-ordering for window
  stack.
- Decouples taskbar from window z-order to provide a stable button
  layout that isn't affected by focus changes.
- Adds culling to the terminal history view to prevent rendering
  off-screen lines.

client/src/debug/debug_overlay.cpp

@@ -0,0 +1,52 @@
+#include <cstdio>
+
+#include "debug_overlay.h"
+#include "ui/ui_renderer.h"
+#include "gfx/types.h"
+
+DebugOverlay::DebugOverlay(void) :
+    _fps(0.0f),
+    _frame_time(0.0f),
+    _draw_calls(0),
+    _shape_vertices(0),
+    _text_vertices(0),
+    _update_timer(0.0f) {}
+
+void DebugOverlay::update(float dt, int draw_calls, int shape_verts, int txt_verts) {
+  _update_timer += dt;
+  /* Only update a few times per sec to keep them readable. */
+  if(_update_timer > 0.2f) {
+    _update_timer     = 0.0f;
+    _frame_time       = dt * 1000.0f;
+    _fps              = 1.0f / dt;
+    _draw_calls       = draw_calls;
+    _shape_vertices   = shape_verts;
+    _text_vertices    = txt_verts;
+  }
+}
+
+void DebugOverlay::render(UIRenderer* ui_renderer) {
+  const Color text_color  = { 0.0f, 1.0f, 0.0f }; /* Bright green. */
+  const float line_height = 18.0f;
+  const float padding     = 5.0f;
+
+  char buffer[256];
+
+  /* Manages its own rendering batch. */
+  ui_renderer->begin_shapes();
+  ui_renderer->begin_text();
+
+  snprintf(buffer, sizeof(buffer), "FPS: %.0f (%.2f ms)", _fps, _frame_time);
+  ui_renderer->render_text(buffer, padding, padding+line_height*1, text_color);
+
+  snprintf(buffer, sizeof(buffer), "Draw Calls: %d", _draw_calls);
+  ui_renderer->render_text(buffer, padding, padding+line_height*2, text_color);
+
+  snprintf(buffer, sizeof(buffer), "Vertices: %d (Shape %d, Text: %d)",
+           _shape_vertices + _text_vertices, _shape_vertices, _text_vertices);
+  ui_renderer->render_text(buffer, padding, padding+line_height*3, text_color);
+
+  ui_renderer->flush_shapes();
+  ui_renderer->flush_text();
+}
+

client/src/debug/debug_overlay.h

@@ -0,0 +1,24 @@
+#pragma once
+
+class UIRenderer;
+
+/**
+ * Renders real-time performance metrics on screen.
+ */
+class DebugOverlay {
+public:
+  DebugOverlay(void);
+
+  void update(float dt, int draw_calls, int shape_verts, int txt_verts);
+  void render(UIRenderer* ui_renderer);
+
+private:
+  float _fps;
+  float _frame_time;
+  int _draw_calls;
+  int _shape_vertices;
+  int _text_vertices;
+
+  /* Oh? You want to be able to read the stats?! */
+  float _update_timer;
+};

client/src/debug/debug_stats.cpp

@@ -0,0 +1,5 @@
+#include "debug_stats.h"
+
+int DebugStats::draw_calls      = 0;
+int DebugStats::shape_vertices  = 0;
+int DebugStats::text_vertices   = 0;

client/src/debug/debug_stats.h

@@ -0,0 +1,14 @@
+#pragma once
+
+struct DebugStats {
+  static int draw_calls;
+  static int shape_vertices;
+  static int text_vertices;
+
+  static void reset(void) {
+    draw_calls      = 0;
+    shape_vertices  = 0;
+    text_vertices   = 0;
+  }
+};
+