ssd: guard against negative sizes

We always create a SSD for 0x0 window since decorations are usually
requested before a window is mapped. Thus the sizes of some buffers/rects
like edge shadows could be negative, which is asserted in wlroots 0.19.

src/ssd/ssd-part.c

@@ -55,15 +55,7 @@ add_scene_rect(struct wl_list *list, enum ssd_part_type type,
 	struct wlr_scene_tree *parent, int width, int height,
 	int x, int y, float color[4])
 {
-	/*
-	 * When initialized without surface being mapped,
-	 * size may be negative. Just set to 0, next call
-	 * to ssd_*_update() will update the rect to use
-	 * its correct size.
-	 */
-	width = width >= 0 ? width : 0;
-	height = height >= 0 ? height : 0;
-
+	assert(width >= 0 && height >= 0);
 	struct ssd_part *part = add_scene_part(list, type);
 	part->node = &wlr_scene_rect_create(
 		parent, width, height, color)->node;