Fix negative dimensions in scene rect sizing

During cross-output tiling drags, computed box dimensions can become
negative due to coordinate transforms and thickness subtractions. This
triggers the assertion in wlr_scene_rect_set_size() which requires
width >= 0 && height >= 0, crashing the compositor.

Clamp dimensions to zero in three locations:
- resize_box() in seatop_move_tiling.c for the WLR_EDGE_NONE case
- update_indicator() in seatop_move_tiling.c as a safety net
- arrange_container() in transaction.c for border width (matching the
  existing vert_border_height clamping from 62fd8c4d)
- update_rect_list() in container.c for pixman region boxes

This is the same class of bug fixed by 62fd8c4d (height clamping) and
partially addressed by c2d6aff6 (too-many-containers guards).

sway/desktop/transaction.c

@@ -435,9 +435,10 @@ static void arrange_container(struct sway_container *con,
 		int border_left = con->current.border_left ? border_width : 0;
 		int border_right = con->current.border_right ? border_width : 0;
 		int vert_border_height = MAX(0, height - border_top - border_bottom);
+		int horiz_border_width = MAX(0, width);
 
-		wlr_scene_rect_set_size(con->border.top, width, border_top);
+		wlr_scene_rect_set_size(con->border.top, horiz_border_width, border_top);
-		wlr_scene_rect_set_size(con->border.bottom, width, border_bottom);
+		wlr_scene_rect_set_size(con->border.bottom, horiz_border_width, border_bottom);
 		wlr_scene_rect_set_size(con->border.left,
 			border_left, vert_border_height);
 		wlr_scene_rect_set_size(con->border.right,
@@ -449,7 +450,7 @@ static void arrange_container(struct sway_container *con,
 		wlr_scene_node_set_position(&con->border.left->node,
 			0, border_top);
 		wlr_scene_node_set_position(&con->border.right->node,
-			width - border_right, border_top);
+			horiz_border_width - border_right, border_top);
 
 		// make sure to reparent, it's possible that the client just came out of
 		// fullscreen mode where the parent of the surface is not the container

sway/input/seatop_move_tiling.c

@@ -81,6 +81,12 @@ static void resize_box(struct wlr_box *box, enum wlr_edges edge,
 		box->y += thickness;
 		box->width -= thickness * 2;
 		box->height -= thickness * 2;
+		if (box->width < 0) {
+			box->width = 0;
+		}
+		if (box->height < 0) {
+			box->height = 0;
+		}
 		break;
 	}
 }
@@ -153,6 +159,12 @@ static bool split_titlebar(struct sway_node *node, struct sway_container *avoid,
 }
 
 static void update_indicator(struct seatop_move_tiling_event *e, struct wlr_box *box) {
+	if (box->width < 0) {
+		box->width = 0;
+	}
+	if (box->height < 0) {
+		box->height = 0;
+	}
 	wlr_scene_node_set_position(&e->indicator_rect->node, box->x, box->y);
 	wlr_scene_rect_set_size(e->indicator_rect, box->width, box->height);
 }

sway/tree/container.c

@@ -338,7 +338,15 @@ static void update_rect_list(struct wlr_scene_tree *tree, pixman_region32_t *reg
 		if (i < len) {
 			const pixman_box32_t *box = &rects[i++];
 			wlr_scene_node_set_position(&rect->node, box->x1, box->y1);
-			wlr_scene_rect_set_size(rect, box->x2 - box->x1, box->y2 - box->y1);
+			int rect_width = box->x2 - box->x1;
+			int rect_height = box->y2 - box->y1;
+			if (rect_width < 0) {
+				rect_width = 0;
+			}
+			if (rect_height < 0) {
+				rect_height = 0;
+			}
+			wlr_scene_rect_set_size(rect, rect_width, rect_height);
 		}
 	}
 }