[client] imgui: track last rectangles for overlays

This is necessary in case overlays change size. When this happens, we must
damage the larger of the overlays' rectangles this frame and last frame.
This erases the overlay from where it is no longer appears.

In order to do this, we must keep track of the rectangles for every overlay
with no exception. We cannot short-circuit the generation of rectangles if
we run out of buffer space, and we must allocate space for MAX_OVERLAY_RECTS
rectangles for every frame. Otherwise, we will not know where to erase the
overlay if it disappears.

client/include/app.h

@@ -80,6 +80,7 @@ void app_glSetSwapInterval(int interval);
 void app_glSwapBuffers(void);
 #endif
 
+#define MAX_OVERLAY_RECTS 10
 void app_registerOverlay(const struct LG_OverlayOps * ops, void * params);
 
 /**