gpu: Make intersecting rounded rect with NONE clip always succeed

This is a tricky topic, because it can make the clip bounds grow, so
previously we were trying to be careful.

However, this can cause perfectly trivial intersections to fail that are
caused by redraw diff regions.
And in the worst case, that means we offscreen in places where we
absolutely do not want to offscreen - in subtrees with subsurface nodes.

Fixes #6499

gsk/gpu/gskgpuclip.c

@@ -30,6 +30,14 @@ gsk_gpu_clip_init_rect (GskGpuClip            *clip,
   gsk_rounded_rect_init_from_rect (&clip->rect, rect, 0);
 }
 
+static void
+gsk_gpu_clip_init_rounded_rect (GskGpuClip           *self,
+                                const GskRoundedRect *rect)
+{
+  self->type = GSK_GPU_CLIP_ROUNDED;
+  gsk_rounded_rect_init_copy (&self->rect, rect);
+}
+
 void
 gsk_gpu_clip_init_copy (GskGpuClip       *self,
                         const GskGpuClip *src)
@@ -129,6 +137,13 @@ gsk_gpu_clip_intersect_rounded_rect (GskGpuClip           *dest,
       break;
 
     case GSK_GPU_CLIP_NONE:
+      res = gsk_rounded_rect_intersect_with_rect (rounded, &src->rect.bounds, &dest->rect);
+      if (gsk_gpu_clip_init_after_intersection (dest, res))
+        break;
+      /* XXX: This may grow the bounds quite substantially */
+      gsk_gpu_clip_init_rounded_rect (dest, rounded);
+      break;
+
     case GSK_GPU_CLIP_CONTAINED:
     case GSK_GPU_CLIP_RECT:
       res = gsk_rounded_rect_intersect_with_rect (rounded, &src->rect.bounds, &dest->rect);