[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/include/app.h

@@ -27,6 +27,7 @@
 #include "common/ringbuffer.h"
 #include "common/types.h"
 #include "interface/displayserver.h"
+#include "interface/overlay.h"
 
 typedef enum LG_MsgAlert
 {
@@ -79,10 +80,15 @@ void app_glSetSwapInterval(int interval);
 void app_glSwapBuffers(void);
 #endif
 
-typedef struct ImGuiGraph * GraphHandle;
+void app_registerOverlay(const struct LG_OverlayOps * ops, void * params);
+int app_renderOverlay(struct Rect * rects, int maxRects);
+void app_freeOverlays(void);
+
+struct OverlayGraph;
+typedef struct OverlayGraph * GraphHandle;
+
 GraphHandle app_registerGraph(const char * name, RingBuffer buffer);
 void app_unregisterGraph(GraphHandle handle);
-bool app_renderImGui(void);
 
 void app_clipboardRelease(void);
 void app_clipboardNotifyTypes(const LG_ClipboardData types[], int count);

client/include/interface/overlay.h

@@ -22,6 +22,8 @@
 #define _H_I_OVERLAY_
 
 #include <stdbool.h>
+#include <assert.h>
+
 #include "common/types.h"
 
 enum LG_OverlayFlags
@@ -57,7 +59,8 @@ struct LG_OverlayOps
    * mode.
    *
    * The caller provides `windowRects` to be populated by the callee and is sized
-   * according to the return value of `getWindowCount`
+   * according to the return value of `getWindowCount`. Note, `windowRects` may
+   * be NULL if the caller does not want this information.
    */
   void (*render)(void * udata, bool interactive, struct Rect windowRects[]);