wayland: fix window resize and populate button modifier state

Three related fixes for the Wayland backend:

wl_handlers.c: populate u.button.state with keyboard modifiers
  Button events were built with state=0 after memset, so all modifier-
  dependent logic in window.c (resizebarMouseDown, frameMouseDown,
  titlebarMouseDown) received no modifier information.  Read the current
  depressed modifier mask from the seat keyboard and copy it directly --
  wlroots WLR_MODIFIER_* bits match the WM_MOD_* layout exactly.

wl_framebuf.c: fix frame_buf_hit to resolve child sub-windows
  frame_buf_at() uses wlr_scene_node_at() which skips frame_buf children
  whose scene_buf has no committed buffer (resizebar, titlebar pixels are
  composited into the parent frame buffer).  The hit always resolved to
  the top-level frame, so events were dispatched to frameMouseDown instead
  of resizebarMouseDown, and resize never started.  After frame_buf_at
  returns a top-level frame, do a coordinate-based child search and return
  the smallest (most specific) child that contains the pointer -- the same
  strategy frame_buf_button_at uses for buttons.

wl_events.c: fix wl_coords_translate to return frame-relative coords
  The stub was a pass-through returning root-relative coordinates unchanged.
  wMouseResizeWindow calls coords_translate(root, frame->core->window, ...)
  to get frame-relative click coordinates for getResizeDirection().  Look up
  the dst window in the frame_buf list and subtract its absolute position.

src/backend/wayland/wl_events.c

@@ -31,7 +31,18 @@ int
 wl_coords_translate(WNativeWindow src, WNativeWindow dst,
                     int src_x, int src_y, int *dst_x, int *dst_y)
 {
-    (void)src; (void)dst;
-    *dst_x = src_x; *dst_y = src_y;
+    (void)src;
+    /* Convert src-window-relative (or root-relative) coordinates into
+     * dst-window-relative coordinates by subtracting dst's absolute position.
+     * Used by wMouseResizeWindow to get frame-relative click coords for
+     * getResizeDirection(). */
+    struct wl_frame_buf *fb = frame_buf_find(dst);
+    if (fb) {
+        *dst_x = src_x - fb->x;
+        *dst_y = src_y - fb->y;
+    } else {
+        *dst_x = src_x;
+        *dst_y = src_y;
+    }
     return 0;
 }

src/backend/wayland/wl_framebuf.c

@@ -300,6 +300,26 @@ frame_buf_hit(int px, int py)
     struct wl_frame_buf *fb = frame_buf_at(px, py);
     if (!fb)
         fb = frame_buf_button_at(px, py);
+
+    /* If we landed on a top-level frame (parent_id==0), check whether a
+     * child sub-window (resizebar, titlebar) actually contains (px,py).
+     * Child frame_bufs may not have a committed scene_buf buffer so
+     * wlr_scene_node_at skips them; we find them by coordinate instead.
+     * Return the smallest (most specific) child that encloses the point. */
+    if (fb && fb->parent_id == 0) {
+        struct wl_frame_buf *child, *best_child = NULL;
+        wl_list_for_each(child, &wl_state.frame_list, link) {
+            if (child->parent_id != fb->id) continue;
+            if (px < child->x || px >= child->x + child->width)  continue;
+            if (py < child->y || py >= child->y + child->height) continue;
+            if (!best_child ||
+                (child->width * child->height) < (best_child->width * best_child->height))
+                best_child = child;
+        }
+        if (best_child)
+            fb = best_child;
+    }
+
     wl_dump_hit_debug(px, py, fb);
     return fb;
 }

src/backend/wayland/wl_handlers.c

@@ -272,6 +272,10 @@ handle_pointer_button(struct wl_listener *listener, void *data)
         case 0x112: wev.u.button.button = 2; break;
         default:    wev.u.button.button = (ev->button - 0x110) + 1; break;
         }
+        {
+            struct wlr_keyboard *kb = wl_state.seat ? wl_state.seat->keyboard_state.keyboard : NULL;
+            wev.u.button.state = kb ? kb->modifiers.depressed : 0;
+        }
         /* For WINGs views, resolve the child widget at click coordinates.
          * Two cases:
          * 1. fb->id >= 0x20000000 — fb IS a WINGs view frame_buf, use directly.