Use damage rectangles for splitting

This is much better when the damage is 'reasonable',
for example the 3-4 rectangles in widget-factory. It
breaks down when we end up with 20 - 30 rectangles,
such as when switching pages in gtk4-demo.

gsk/ngl/gsknglrenderer.c

@@ -174,15 +174,14 @@ get_render_region (GdkSurface   *surface,
   damage = gdk_draw_context_get_frame_region (GDK_DRAW_CONTEXT (context));
 
   if (cairo_region_contains_rectangle (damage, &whole_surface) == CAIRO_REGION_OVERLAP_IN)
-    return NULL;
+    return cairo_region_create_rectangle (&whole_surface);
 
   /* If the extents match the full-scene, do the same as above */
   cairo_region_get_extents (damage, &extents);
   if (gdk_rectangle_equal (&extents, &whole_surface))
-    return NULL;
+    return cairo_region_create_rectangle (&whole_surface);
 
-  /* Draw clipped to the bounding-box of the region. */
-  return cairo_region_create_rectangle (&extents);
+  return cairo_region_copy (damage);
 }
 
 static void
@@ -196,6 +195,7 @@ gsk_ngl_renderer_render (GskRenderer          *renderer,
   GskNglRenderJob *job;
   GdkSurface *surface;
   float scale_factor;
+  gint64 start_time, end_time;
 
   g_assert (GSK_IS_NGL_RENDERER (renderer));
   g_assert (root != NULL);
@@ -215,15 +215,53 @@ gsk_ngl_renderer_render (GskRenderer          *renderer,
   render_region = get_render_region (surface, self->context);
 
   gsk_ngl_driver_begin_frame (self->driver, self->command_queue);
+
+  start_time = g_get_monotonic_time ();
+  g_print ("tiled:");
+  end_time = start_time;
+
+  for (int i = 0; i < cairo_region_num_rectangles (render_region); i++)
+    {
+      cairo_rectangle_int_t rect;
+      cairo_region_t *region;
+      gint64 st = end_time;
+
+      cairo_region_get_rectangle (render_region, i, &rect);
+      region = cairo_region_create_rectangle (&rect);
+
+      job = gsk_ngl_render_job_new (self->driver, &viewport, scale_factor, region, 0);
+#ifdef G_ENABLE_DEBUG
+      if (GSK_RENDERER_DEBUG_CHECK (GSK_RENDERER (self), FALLBACK))
+        gsk_ngl_render_job_set_debug_fallback (job, TRUE);
+#endif
+      gsk_ngl_render_job_render (job, root);
+      gsk_ngl_render_job_free (job);
+
+      cairo_region_destroy (region);
+
+      end_time = g_get_monotonic_time ();
+
+      g_print ("%s %.2f", i > 0 ? " /" : "", (end_time - st) / 1000.0);
+    }
+
+  g_print (" ms");
+
+  start_time = g_get_monotonic_time ();
+
   job = gsk_ngl_render_job_new (self->driver, &viewport, scale_factor, render_region, 0);
 #ifdef G_ENABLE_DEBUG
   if (GSK_RENDERER_DEBUG_CHECK (GSK_RENDERER (self), FALLBACK))
     gsk_ngl_render_job_set_debug_fallback (job, TRUE);
 #endif
   gsk_ngl_render_job_render (job, root);
-  gsk_ngl_driver_end_frame (self->driver);
   gsk_ngl_render_job_free (job);
 
+  end_time = g_get_monotonic_time ();
+
+  g_print ("    non-tiled: %.2fms\n", (end_time - start_time) / 1000.0);
+
+  gsk_ngl_driver_end_frame (self->driver);
+
   gdk_gl_context_make_current (self->context);
   gdk_draw_context_end_frame (GDK_DRAW_CONTEXT (self->context));