vk: include boundary images in the blur-behind snapshot

An ImageQuad whose z_before equals a batch's start vertex was drawn
inside that batch's interleave loop — for a blur-behind batch that
means after snapshot_frame_so_far, so the plate's blur never sampled
it. cce-browser hit this exactly: the full-window page image sorts
right at the floating bar's plate batch boundary, and the bar frosted
only the clear color beneath it.

Flush images due at each batch boundary before the snapshot (and mark
the shared snapshot stale when they draw, matching the non-blur-draw
invalidation rule).

Co-Authored-By: Claude Fable 5 <[email protected]>

 src/vk/renderer.rs | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/src/vk/renderer.rs b/src/vk/renderer.rs
index 55f9b48..a7a9423 100644
--- a/src/vk/renderer.rs
+++ b/src/vk/renderer.rs
@@ -1769,6 +1769,31 @@ impl VkRenderer {
                 let mut snapshot_fresh = false;
 
                 for batch in batches {
+                    // Images due at this batch's boundary draw first: they sit
+                    // beneath the batch's geometry, and a blur snapshot taken
+                    // for this batch must capture them (an image whose
+                    // `z_before` equals the batch start would otherwise slip
+                    // to after the snapshot and never be frosted).
+                    while let Some(&k) = order.get(img_i) {
+                        let q = &images[k];
+                        if q.z_before > batch.start {
+                            break;
+                        }
+                        img_i += 1;
+                        let img_scissor = match q.clip {
+                            Some((cx, cy, cw, ch)) => vk::Rect2D {
+                                offset: vk::Offset2D { x: cx as i32, y: cy as i32 },
+                                extent: vk::Extent2D {
+                                    width: cw.min(self.extent.width.saturating_sub(cx)),
+                                    height: ch.min(self.extent.height.saturating_sub(cy)),
+                                },
+                            },
+                            None => full_scissor,
+                        };
+                        self.core.device.cmd_set_scissor(cmd, 0, &[img_scissor]);
+                        self.image.record_quad(&self.core.device, cmd, frame_index, k, q.image);
+                        snapshot_fresh = false;
+                    }
                     if batch.blur_behind {
                         if !snapshot_fresh {
                             self.snapshot_frame_so_far(cmd, image_index as usize);