Decoration: fix clip x/y coordinates

We need to adjust for the decoration offset since the clip applies
relative to the root of the decoration subsurface tree but is defined
relative to the window.

 river/Decoration.zig | 10 +++++++++-
 river/Window.zig     |  6 +-----
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/river/Decoration.zig b/river/Decoration.zig
index f73ad29..4e432ce 100644
--- a/river/Decoration.zig
+++ b/river/Decoration.zig
@@ -137,7 +137,7 @@ fn commit(wlr_surface: *wlr.Surface) callconv(.c) void {
     }
 }
 
-pub fn renderFinish(decoration: *Decoration) void {
+pub fn renderFinish(decoration: *Decoration, window_clip: *const wlr.Box) void {
     const rendering_requested = &decoration.rendering_requested;
     if (rendering_requested.sync_next_commit) {
         rendering_requested.sync_next_commit = false;
@@ -154,4 +154,12 @@ pub fn renderFinish(decoration: *Decoration) void {
     decoration.surfaces.dropSaved();
 
     decoration.tree.node.setPosition(rendering_requested.offset_x, rendering_requested.offset_y);
+
+    // wlroots asserts that a subsurface tree is present.
+    if (!decoration.surfaces.tree.children.empty()) {
+        var clip = window_clip.*;
+        clip.x -= rendering_requested.offset_x;
+        clip.y -= rendering_requested.offset_y;
+        decoration.surfaces.tree.node.subsurfaceTreeSetClip(&clip);
+    }
 }
diff --git a/river/Window.zig b/river/Window.zig
index bd70c98..3cbc177 100644
--- a/river/Window.zig
+++ b/river/Window.zig
@@ -879,11 +879,7 @@ pub fn renderFinish(window: *Window) void {
     inline for (.{ &window.decorations_above, &window.decorations_below }) |decorations| {
         var it = decorations.iterator(.forward);
         while (it.next()) |decoration| {
-            decoration.renderFinish();
-            // wlroots asserts that a subsurface tree is present.
-            if (!decoration.surfaces.tree.children.empty()) {
-                decoration.surfaces.tree.node.subsurfaceTreeSetClip(&window.rendering_requested.clip);
-            }
+            decoration.renderFinish(&window.rendering_requested.clip);
         }
     }
 }